The number of test users you need to create depends on the workflows you plan to test in your application. Because transactions have both buyers and sellers, you should create at least one seller and one buyer to test most flows. You need both because eBay doesn’t allow a seller to purchase or bid on their own listing, a rule that also applies in the Sandbox.
If you’re going to test multiple-quantity, fixed-price listings or auction listings and competitive bidding between multiple buyers, create at least one seller and at least two buyers.
Even if your application only supplies functionality that serves sellers, such as only listing items with no bidding or purchasing, you should also test the effects of buyer actions to make sure that your application handles all of the interactions it may encounter when in Production. We recommend testing your application for typical scenarios such as users bidding on items, users winning bids, the calculation of shipping costs, and a non-paying bidder.