sellers.json is the ad-system side of supply-chain transparency. Where a publisher’s app-ads.txt says which ad systems may sell its inventory, an ad system’s sellers.json says who is behind each seller account it carries.
Buyers cross-check the two files to confirm that an account ID in a publisher’s app-ads.txt belongs to who it claims to, and to see whether a seller is the publisher itself or an intermediary.
For a publisher, the practical point is simple: keep your app-ads.txt accurate, because that is the half of the pair you control. See the app-ads.txt reference for what real files look like.