I had such an experience too...And it really ticks me off as well. But guess what, if you leave them a bad feedback because of non-paying sales, they might do the same to you too! You know...some bad buyers are simply unreasonable. They don't even bother to reply to you or anything. Give them 48hrs with 1 or 2 notices, if they did not respond, just post the domains back on sale. If someone came and buy your domain and that non-paying buyer confronted you about it, just tell him/her be glad that you did not even leave him/her a bad feedback for not paying. And of course, DO post in the public the non-paying buyer's username stating clearly he/she refuse to pay for the domain. I think that's fair and also, protect you from having unjustified ratings.