Ads.txt is the IAB Tech Lab’s transparency that increases visibility into which companies are allowed to sell a given publisher’s inventory. Specifically, it allows publishers to “publicly declare” or list authorized sellers in an easily updated and maintained txt file. 

The main difference is that ads.txt is provided by the publisher and lists who is authorized to sell their inventory. In contrast, sellers.json is a json file provided by the advertising platform(i.e. exchanges and SSPs) that identifies the sellers it transacts with. 

In other words, ads.txt is a list of who can sell an impression. Sellers.json and SupplyChain object uncover who did participate in the sale of a given impression.