Supply chain
sellers.json
sellers.json is a public file in which SSPs and exchanges list their seller accounts, naming who owns each one and whether it is a publisher or an intermediary.
If ads.txt is the publisher saying "these accounts may sell my inventory", sellers.json is the SSP saying "this is who owns each account". The file, hosted at the ad system's domain (for example ssp.example/sellers.json), lists every seller ID with a name, domain and seller type: PUBLISHER, INTERMEDIARY or BOTH. Some entries can be marked confidential.
The IAB Tech Lab released sellers.json in 2019 together with the supplyChain object. Together they let a buyer trace who touched a bid request: the schain lists each seller ID in the path, and sellers.json reveals the company behind each ID.
For buyers practising supply path optimization, sellers.json exposes unexpected intermediaries, resellers of resellers, and accounts belonging to known traffic arbitrage or made-for-advertising sites operators. Its weaknesses are incomplete or confidential entries and inconsistent data quality, which is why buyers treat missing or opaque entries as a risk signal.
Think of it like this
sellers.json is like the membership register at a trading market: it tells you which stall number belongs to which real business.
An example
A bid request's schain shows seller ID 5501 at an SSP. The SSP's sellers.json lists 5501 as "Example Media Ltd, INTERMEDIARY", revealing a reseller rather than the publisher itself.
Related terms
ads.txt
ads.txt (Authorized Digital Sellers) is a public text file on a website's domain listing which ad sellers and accounts are authorised to sell that website's ad inventory.
SupplyChain object (schain)
The SupplyChain object (schain) is a field in OpenRTB bid requests that lists, in order, every seller or intermediary that handled the impression on its way from the publisher to the buyer.
Supply path optimization (SPO)
Supply path optimization (SPO) is a buyer's practice of choosing the most direct, cheapest and most trustworthy routes to the same inventory, and cutting unnecessary resellers and duplicate paths.
Reseller
A reseller is a company authorised to sell a publisher's ad inventory through an ad system account that it controls, rather than the publisher's own account, and labelled RESELLER in ads.txt.
Direct seller
A direct seller is an ad system account that the publisher itself controls and gets paid through directly, marked DIRECT in the publisher's ads.txt file.
Sources: IAB Tech Lab: sellers.json