This is one thing that bugs me about ebay.
If you are not totally in the wrong and leave another member a negative feedback on a bad item (when you are not at fault) in most cases, because of the nature of most people, the natural reaction is to retaliate, so they'll give you a negative feedback in return. If you've got a perfect feedback record, you may end up getting one bad feedback and ruining the perfect record, even though you've really done nothing wrong, which sucks. I mean, you can appeal feedbacks to ebay, sure, but it's a pain in the butt.