The one that annoys me is tit-for-tat feedback - someone's a crap buyer/seller but gets legitimate bad feedback so retaliates. That's where my two negatives come from!

If you want to see an "interesting" tit-for-tat ebayer, look at umb61. They sell crap - badly printed and bound photocopies of old magazine reports which they claim are "reference books". And if you point out that their stuff is crap, they leave vituperative negative feedback.