I think I read that if you have a buyer that doesn't pay and leaves negative feedback, you can get the feedback taken off somehow. You're just stuck with a temporary negative.
The real problem I can see is people with private feedbacks going around and leaving negatives for fun. I doubt it will happen, but sellers would never know until they're hit with a negative.
Personally, I'd be more than willing to take my business to another eBay-like site that doesn't pocket so much of my money. But there just doesn't seem to be any decent alternatives, at least not that I know of.