The accounts' transaction histories, and the transaction histories of their transactants is all that anyone - even Amazon - has to go on - in both toy scenarios

The accounts' transaction histories, and the transaction histories of their transactants is all that anyone - even Amazon - has to go on - in both toy scenarios. The difference here is the transparency, and the resulting web of trust that can emerge. You may not have done business with company X, but you can observe that many users who successully transacted with companies Y, Z and W (with whom you have also successfully transacted) have all left good purchase reviews for company X.