Was the buyer who left the bad feedback fairly new, OR had a very low feedback himself? You used to be able to block buyers with a feedback score smaller than a certain amount. People with larger scores tend to be more stable mentally..
I used to sell on Ebay all the time, and basically gave it up due to all the scammers that have taken over. I sold something a couple months ago out of the blue, it was over $500 "buy it now" ,and the buyer only registered a couple minutes before bidding, and had ZERO feedback. I almost crapped my pants every day for a week before he picked it up from his post office! I thought for SURE he'd end up with it, and my money, and I'd be screwed. In his case though he exhibited every sign of a scammer, he was legit, lucky for me. Still scary though.