My last job was as the customer service manager for a company that sold computers on
ebay, and I dealt with customers like that every day. It's hard not to take things like this personally, but that's what you have to be able to do to be successful on
ebay. The real problem comes from people who leave negative feedback, which can have a serious impact on your sales. We always bent over backwards to get positive feedback (or get negative feedback changed to positive), even when the buyer was being unreasonable. Then we blocked the problem buyer. As a result, we had excellent feedback and Dealer Seller Ratings, which earned us a lot of sales. The "don't take it personally" thing was always one of the hardest things for my customer service reps to learn, but it will pay dividends on your bottom line.