What is sellers.json?
sellers.json is a file that SSPs and ad exchanges publish listing the publishers and intermediaries they pay, with each seller's account ID, name, domain and whether it is a publisher or an intermediary. Buyers match it against ads.txt to see who is really behind an impression. The IAB Tech Lab released the standard in 2019.
sellers.json closes a gap in ads.txt: ads.txt lists account IDs, and sellers.json tells you whose accounts they are. If an account is marked confidential, or labelled an intermediary, buyers know they are dealing with a reseller rather than the publisher itself.
With the supplyChain object, which lists every seller that handled a bid request, buyers can reconstruct the full path and verify each hop. This transparency underpins supply path optimization and helps expose hidden traffic arbitrage.