as for the A500 - right, it was sold 'as-is', how does it look to you? here's a link to the auction:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=004&sspagename=STRK%3AMESO%3AIT&viewitem=&item=140097078675&rd=1&rd=1
I hope you wrote the serial number of that A500 down just in case the buyer tries to do a "switch" on you and return the defective A500 he previously bought.
Also, make he knows what he is doing in hooking up the monitor. It's unfortunate that sometimes people have to pay for return shipping costs because the buyer doesn't have a clue / didn't know what he or she was getting onto.
I think since you said it was "AS-IS" you should have to refund his or her money, but you're right that there is a serious Ebay flaw with retaliatory feedback.
I think there should be a double blind feedback posting system on Ebay. Feedback should be stored by Ebay's computers and NOT posted publically until BOTH the buyer and seller have submitted their feedback to Ebay --- then both "feedbacks" are posted simultaneously. Only then could the buyer and seller read the other person's feedback.
