You co the sensible thing and cancel the sale, then. no selling fee to pay, and add "buyer" to your. locked bidders list.
Getting involved with this sort of buyer only causes aggravation for sellers.
Didn't you read the message sent to members about how ebay is going to make it "easier" to do this. As far as I can see they are not really changing anything that sensible sellers would not do in any case.
You cannot leave negative comments on positive feedback, it can be removed and would cause you more trouble than the buyer.