ads.txt, sellers.json and the SupplyChain object work together to prove a request came through an authorized path. ads.txt lists which exchanges and account IDs are allowed to sell a domain; sellers.json lets buyers look up who a seller ID belongs to; and the SupplyChain object records the path a request actually travelled.
The most common causes of an unauthorized-seller flag are a missing or mistyped ads.txt line for the exchange, a publisher account ID that does not match, a DIRECT vs RESELLER relationship error, or a SupplyChain seller ID that is not present in the exchange's sellers.json. Any of these breaks the chain of trust and can stop spend even when targeting looks open.
To fix it, add the exact line the exchange specifies to your ads.txt, confirm the account ID matches your sellers.json entry, and make sure the SupplyChain node uses the same authorized seller ID. Then re-test with a sampled bid request to confirm the buyer now treats the path as authorized.