one is a Green with a 'non-payer' note   Which has, incidentally, given him another star.

You can block a buyer if they have -1 star or worse. You can also block someone if they've bought at least 1 item in the last 10 days and have 5 stars or less. Apart from that, the stars don't seem to matter much in terms of automatic blocking. As kopenhagen5 said, the blocks for receiving a strike through UPI are generally pretty good. Most buyers do seem to pay but of course the larger the store the more odds are of more non-payers.