A buyer is the entity that pays for an advertising impression. In programmatic trading the word is narrower than in ordinary commerce: it names an account, registered in advance with an exchange or a supply-side platform, against which spend is billed, policy is enforced and blocking decisions are applied. Google's Real-time Bidding API documentation states the split plainly. Bidders place bids for ad inventory on behalf of one or more buyers. Buyers place ads and are billed for placements. The same company can be both, and frequently is, but the protocol keeps them separate because the money and the machinery do not always belong to the same party.

A demand-side platform (DSP) holds one connection to an exchange and routes the spend of hundreds of agencies and brands through it. The exchange still needs to know which of those hundreds sits behind a given bid, because floors, deal terms, category blocks and invoices attach to the payer rather than the pipe.

How a buyer appears in the bid stream

OpenRTB, the IAB Tech Lab specification governing most automated media transactions, identifies buyers by seat. Two arrays sit at the top of the bid request. According to the 2.6 specification, released in April 2022, wseat carries an allowed list of buyer seats permitted to bid, bseat a block list. At most one should appear in a request, and omitting both implies no restriction. Seat IDs, and knowledge of the buyer customers behind them, must be coordinated between bidder and exchange beforehand. The same wseat array reappears inside the Deal object, which is how a private marketplace agreement is fenced off for named buyers.

On the return leg the response splits into SeatBid objects, each carrying a seat string and one or more Bid objects. Version 2.3.1, published in November 2014, described the seat as the buying entity upstream from the bidder, and noted that one seat may submit several bids from different campaigns to improve its chances where sellers enforce block lists.

Seat identity then does real work. IAB Tech Lab's Programmatic Auction Definitions, finalized on June 26, 2026, place it at step seven of a twelve-step workflow: qualifying received bids means checking the bid's seat and advertiser against seller allow and block lists covering categories, ad domains, buyer seats and creative IDs. The same document defines a buying platform as an entity that evaluates impression opportunities and offers to purchase them, keeping platform and payer conceptually apart.

A second field carries the buyer's view of the audience rather than its identity. The User object's buyeruid is defined as a buyer-specific ID for the user as mapped by the exchange for the buyer; on the web it has conventionally held the DSP's own cookie value, retrieved through a prior cookie sync.

Google layers its own accounting on top. In its OpenRTB integration guide, the imp.ext.billing_id extension carries the billing IDs of eligible buyers and matched pretargeting configurations, with a parallel field inside each deal. Those IDs attribute the winning impression for billing, reporting, policy and block enforcement.

Seats, members and account hierarchies

Account structures differ by platform, and that is where confusion starts. Microsoft's Monetize documentation for external DSPs using buyer seat IDs records the migration bidder by bidder: The Trade Desk, Display and Video 360, Yahoo DSP, Zeta and StackAdapt now transact on seats rather than platform member IDs, which run to four to six digits. Seat IDs take whatever form the DSP chooses and are unique only within that DSP, so two unrelated buyers on two platforms can share a string. A seat becomes visible to sellers when the DSP registers it, or the first time it bids.

The seat is also the unit of commercial negotiation. Buyer accounts on OpenX are identified by seat IDs, the level at which certain commercial terms apply, including automated partner discounts. When LinkedIn opened profile-derived segments to connected television, the buyer had to supply a DSP seat name and seat ID before a deal ID could be generated.

Google's hierarchy nests buyers under bidders, addressing a buyer as buyers/{accountId} and its bidder as bidders/{bidderAccountId}. A child seat, according to Authorized Buyers help documentation, is typically an agency holding its own account, which it may open to a DSP through a sign-in. Parent seats see bid compliance metrics aggregated across their own deals and their children's.

Origin and evolution

The vocabulary comes from securities exchanges, where a seat conferred the right to trade on the floor. An entity buys the right to bid, and its seat is the name under which bids arrive. OpenRTB codified it early: version 1.0 of the block list specification appeared in December 2010, and the seat arrays have survived every 2.x release since, with 2.6 replacing whitelist and blacklist wording with allowed list and block list.

Google renamed its side of the market on June 27, 2018, when the DoubleClick brand was retired. Buyers and networks previously called AdX buyers became Authorized Buyers, a change framed as reflecting the closeness of the publisher relationship. The protocol took longer: Google sunset its proprietary Authorized Buyers bidding protocol on February 15, 2025, moving all bid requests to OpenRTB.

Buyer identity gained its own transparency standard in 2021. IAB Tech Lab released buyers.json and the DemandChain object for public comment on March 30, 2021, with the window closing on April 30 and version 1.0 following in June. The pair mirrors sellers.json and the SupplyChain object, unveiled in July 2019 and adopted quickly on the sell side after exchanges including Index Exchange implemented them. buyers.json lets an advertising system declare the buyers it represents; DemandChain names every entity involved in a bid response. The specification uses a bsid attribute to identify who is paying the advertising system to serve a creative, requires each bsid to map to exactly one paying entity, and acknowledges that one buying entity may hold several seat IDs in a single system.

Why buyer identity matters to marketers

Because policy and blocking attach to the buyer, the account determines what a campaign may lawfully run. Google replaced the ad networks blocking control in AdSense with an Authorized Buyers control on November 6, 2025. A blocked buyer cannot purchase the inventory, and its bids do not execute, though the inventory stays visible in its targeting tools.

Policy has diverged by channel. Google told publishers in December 2025 that Authorized Buyers could promote prescription drugs in certain markets without Google certification, a requirement that still binds advertisers buying through Google Ads. Gambling split the same way: end-advertiser certification was removed on the Authorized Buyers side from August 10, 2026 while direct-buy rules tightened weeks later. One advertiser, two gates, depending on which buyer account routes the spend.

Economics attach to the buyer too. Citing the ANA Programmatic Transparency Benchmark 2025, IAB Spain's supply-side platform guide reported that only 41 percent of programmatic investment reaches genuine, measurable, viewable impressions, with 26.1 percent consumed by DSP fees, data costs and SSP costs. Take rates vary: reporting cited by PPC Land has placed Amazon DSP at roughly 1 to 2 percent of media spend against 12 to 15 percent for The Trade Desk.

Disambiguation

Buyer and bidder. The bidder is the technical endpoint receiving traffic and returning responses. The buyer is the billed account. Google models them as separate resources that share an ID only when a bidder buys on its own behalf.

Buyer and advertiser. The advertiser is the brand whose product appears in the creative, declared through the adomain field. The buyer may be an agency, trading desk or reseller several steps removed.

Buyer seat and deal ID. A seat identifies who is buying; a deal ID identifies the terms under which a package of inventory is sold. Deals restrict access by seat through wseat, but the two are not interchangeable.

Buyer and media buyer. In agency usage the word names a person who plans and executes purchases. Nothing in OpenRTB refers to that role.

Limitations and disputes

Buy-side transparency has not matched the sell side. Research covered by PPC Land found ads.txt hosted by at least 72.64 percent of European web publishers; buyers.json and DemandChain need synchronized adoption across intermediaries before producing anything useful, and have not achieved it.

The buyeruid field has drawn a sustained dispute. IAB Tech Lab acknowledged in December 2024 that many buyers were surprised the field no longer exclusively carried the result of a one-to-one cookie sync, values instead arriving from other methods, a practice known as ID bridging. Its response was provenance signaling rather than prohibition, leaving the disagreement about acceptable inference unresolved.

Seat restrictions are unevenly implemented. Prebid Server, the open-source header bidding component, returns an HTTP 400 for requests defining wseat or bseat, offering impressions to a bidder only when a bidder-specific extension object is present.

Admission is contested too. Teads alleged in litigation covered by PPC Land that Google admitted Outbrain's Zemanta and Amplify as authorized buyers and then banned them after Teads allowed first-party demand to buy through those platforms. Google has not accepted that account.

Recent developments

Autonomous software is redefining the buyer again. Magnite ran its first agentic test inside SpringServe in December 2025 with Scope3 acting as buyer agent, using the Ad Context Protocol. Evidence on performance is mixed. DataBeat's June 2026 report found conventional programmatic buyers held a 13.4 percent CPM advantage over AI agents, closer to parity than early vendor claims implied. Draft Digital went further, collapsing twelve protocol calls into a single buyer agent, concentrating decisions inside a system that is hard to audit. Sell-side platforms are courting buyers directly, as in InMobi's Buyer Hub.

Timeline

  • December 2010: Version 1.0 of the OpenRTB block list specification is released
  • November 2014: OpenRTB 2.3.1 describes wseat as a whitelist of buyer seats and defines the seat as the buying entity upstream from the bidder
  • June 27, 2018: Google retires the DoubleClick brand and renames AdX buyers as Authorized Buyers
  • July 2019: IAB Tech Lab unveils sellers.json and the SupplyChain object
  • March 30, 2021: buyers.json and the DemandChain object open for public comment, closing April 30, 2021
  • June 2021: DemandChain object version 1.0 published
  • April 2022: OpenRTB 2.6 released, restating wseat and bseat as allowed and block lists
  • December 2024: IAB Tech Lab publishes guidance on identifier provenance following disputes over buyeruid
  • February 15, 2025: Google sunsets its proprietary Authorized Buyers bidding protocol in favor of OpenRTB
  • November 6, 2025: AdSense replaces its ad networks blocking control with an Authorized Buyers control
  • December 2025: Magnite runs its first agentic test with Scope3 as buyer agent
  • June 26, 2026: IAB Tech Lab publishes final Programmatic Auction Definitions, placing buyer seat checks at step seven
  • August 10, 2026: Google removes end-advertiser certification for gambling content on Authorized Buyers in an expanded market list

Summary

Who: Advertisers, agencies, trading desks, ad networks and resellers hold buyer accounts. Demand-side platforms including The Trade Desk, Display and Video 360, Amazon DSP, Yahoo DSP and StackAdapt operate as bidders on their behalf. Exchanges and supply-side platforms including Google's Authorized Buyers marketplace, Microsoft Monetize, OpenX and Magnite register and police those accounts. IAB Tech Lab maintains the specifications that define them.

What: The billed party in a programmatic transaction, identified by a seat ID and named in the wseat and bseat arrays of an OpenRTB bid request, in the seat attribute of each SeatBid object in the response, and in platform-specific fields such as Google's billing IDs and the bsid attribute of the DemandChain object.

When: The concept entered digital advertising with the exchange model in the late 2000s and was codified through OpenRTB from December 2010 onward. Google renamed its buyer population Authorized Buyers on June 27, 2018. Buy-side identity standards arrived in 2021 and remain lightly adopted.

Where: Across open auction, private marketplace and programmatic guaranteed transactions in display, video, audio, connected television and digital out-of-home, wherever OpenRTB or a comparable protocol governs the exchange between selling and buying platforms.

Why: Sellers need to know who is paying in order to enforce block lists, apply deal terms, price inventory differently by counterparty and invoice correctly. Buyers need a stable identity to negotiate access to curated supply. Regulators, auditors and standards bodies need it to trace where advertising money goes.