One of my favorites was a buyer who left negative feedback, only because I did not leave THEM positive feedback. I used to sell many items, and would do feedback in clumps, like about 20 at a time. Besides, I don't HAVE to LEAVE feedback at all if I don't want to. That was years back.
I also had a guy buy an item, and not pay for it, and then HE listed the same item. By the time I was able to relist the item, it was too late to sell it for the holidays. I had to wait for a weeks for it to play out. It was an item worth over $350. He was really sketchy, and did it under different screen names, but I followed his trail. He even EMAILED me, and said, haha, I screwed you. He was able to do this AOL to AOL email, so Ebay said they could not track it back or something. (this also was years ago, and I think AOL email is trackable now).
Oh well.