I've had pretty good luck on eBay, and when I haven't, good manners have typically gone a long way. I recently sold my underwater camera setup, but it took two eBay listings to sell it because the first guy misread the post and backed out after he had already won the auction. However, he offered to do "the honorable thing" and purchase the unit anyway, because he figured that it was his own fault for having misread the listing. I relisted the item instead, but because he was so willing to do whatever it would take to fix his mistake, we both were left feeling warm and fuzzy inside.
I've been an ass only once on eBay, and it was totally unintentional. The guy who backed out after winning my LCD monitor auction (because he thought it was a "plasma TV") wrote me hatemail after I got my eBay auction fee back by filing a non-paying bidder complaint (which at that time was the only way I knew how to get money back). I didn't know then that there was a mutual cancellation option, and he ended up with bad feedback.
Bad things do happen on eBay, and I've been lucky to have avoided them so far. The worst I've had to deal with are rude people, like this guy, who, after waiting only two and half full business days from auction close for me to ship out the CD-ROM drive he won, replied with two words when I emailed him the tracking number:
From: Kurt Fankhauser
Sent: Friday, February 15, 2002 7:43 PM
To: Eric Cheng
Subject: Re: TrueX shipped today
What a rude bastard. Of course, he did pay immediately after auction close, which makes him a good eBay buyer. It's just that he turned out to be an ass, which leaves me with negative feelings about the transation. I'll bet he finds this and writes me hatemail.
I'm going home tomorrow, very early in the morning. Super Shuttle comes three hours early, at the latest. argh. My plans tonight were also cancelled, but I guess I didn't feel like going out, anyway.