OpenBazaar uses a three-way transaction. Buyer, seller, mediator. Only two keys are required to release or revert the transaction. In the event of a dispute The mediator is the final judge, they're supposed to decide based on evidence whether to release th payment to the seller or refund the buyer. I do not recall off-hand what the selection criteria is for the mediators, but I believe that it's not in the control of either party.