Sellers.json is a file hosted by Supply Side Platforms (SSPs) that list the publishers or vendors that are either direct sellers of or intermediaries in the selling of digital advertising. 

While publishers need to have a file ads.txt on their domains, listing the SSPs allowed to sell ads on that domain, the SSPs need to have a also file – sellers.json – listing the publishers they are working with, their business name, their domain, seller type information, and respective id. 

See here the Index Exchange sellers.json

With the sellers.json information, DSPs can then provide more transparency to advertisers, by letting buyers know to which publishers they are buying ads, and also providing the possibility to use the sellers.json as targeting.