They have it, its called Feedback, and if a Seller wont leave appropriate feedback to warn others because of fear of retaliation, then he is not a very honest seller.
Feedback is the tool that reflects how people sell and how they buy. There are reasons why some people dont complete a deal, I know that I backed out of a deal recently and would not want an automatic label stamped next to my ID, as I worked it out with the vendor. (In a nutshell the seller was trying to charge me almost triple the shipping rate, an amount that was over twice what I was paying for the item).
So in this instance, I got out of purchasing the item I won and neither of us (seller or myself) left the other any feedback, something that we mutually agreed upon.
Any other system may have placed a mark beside my ID which would not have been a proper reflection of why I chose not to complete the transaction.
Malcolm