I would give them a couple of strikes then out.
Both the non-payers, and the out-of-stock sellers. It is possible to have a problem paying (accident, for example), and sellers can make a genuine mistake with stock levels, or find an issue with an item at the last minute. Over and over...not acceptable.
-------------------------------------------------------------------------------------------------------------------------------
Buttercup: You mock my pain! Man in Black: Life is pain, Highness. Anyone who says differently is selling something.
