If you are Intermediary or Both:
In case you have been classified as “Intermediary” or “Both” publisher you should use sellers.json to increase transparency in the SupplyChain, revealing all entities taking part in an auction process. This procedure allows DSPs to look up existing connections between sellers and their publishers, as long as both sides curate their respective documentations. Similar to DSPs looking up existing partnerships on ads.txt, the entries in sellers.json can be compared to the information provided in the Supply Chain object, in order to verify an existing relationship between publisher and seller.
If you are Direct Publisher:
Direct publishers don’t have to implement anything. Unlike ads.txt which publishers hosted, sellers.json file needs to be hosted by SSPs/Exchanges and intermediary resellers. It is the responsibility of SSPs/Exchanges/Resellers to implement both Sellers.json and OpenRTB Supply Chain Object