Programmatic · also called principal media, principal trading, arbitrage buying
Principal-based buying
Principal-based buying is when an agency buys media for its own account and resells it to clients at a price it sets, instead of buying at cost as the client's agent.
Traditionally an agency acts as an agent: it buys media on the client's behalf, passes on the actual cost and charges a fee. In principal-based buying, the agency (often through a holding company trading unit) acts as the principal: it buys inventory upfront or commits volume with media owners, then sells it to clients at its own price. The difference between buying and selling price is agency margin.
Agencies argue principal buying lets them secure lower prices through scale and take on risk, and that clients get a good net price. Critics point out that clients usually cannot see the underlying cost, so they do not know how much margin is being made, and the agency's incentive may shift toward selling inventory it owns rather than what best fits the client.
Principal media is legal and increasingly common, but industry bodies such as the ANA advise advertisers to opt in explicitly, cap the share of budget used and demand disclosure. It sits alongside media rebates as a core issue in agency transparency.
Think of it like this
Principal-based buying is like a travel agent who buys hotel rooms in bulk and resells them to you as a package: it might be a good deal, but you do not see what they paid.
An example
An agency commits US$10 million to a streaming service at US$20 CPM and resells the impressions to clients at US$26 CPM, earning a US$6 per-thousand margin.
Related terms
Agency
An agency is a company that plans, buys, creates or manages advertising on behalf of advertisers, in exchange for fees, commissions or a margin on the media it buys.
Holding company (agency holdco)
An agency holding company (holdco) is a parent group that owns many advertising, media, creative and marketing agencies, such as WPP, Publicis Groupe, Omnicom, Dentsu and Havas.
Media rebates
Media rebates are payments, credits or free inventory media owners give agencies for the volume of client spend they bring, which may or may not reach advertisers.
Trading desk
A trading desk is a specialist team, usually inside an agency or advertiser, that plans and executes programmatic buying on DSPs on behalf of campaigns and clients.
Traffic arbitrage
Traffic arbitrage is buying cheap visitors, often through native, social or search ads, and sending them to pages loaded with ads that earn more than the traffic cost.