Know when to hold 'em, and know when to leave 'em.
"The trouble with the global village is all the global village idiots."
The biggest flaw (and in some ways, the biggest strength) of eBay's feedback system is the risk of retaliation. You leave negative or neutral feedback for someone, and they will ? without considering the circumstances or who's at fault ? do the same for you. That is the fear, and that is the reason why many people simply let problems slide.
But the risk of retaliation also reminds people that they are responsible for their own words; if there were no consequences, people would leave negative feedback with abandon, and we'd have even more problems on our hands.
I won't deny that the risks sometimes outweigh the gains. Sometimes a bidder has a seemingly legitimate reason for not paying. Perhaps a seller is inexperienced, and while a particular transaction might not have gone very smoothly, it wasn't due to any malice by the seller. Do these people necessarily deserve blemishes on their records? Perhaps not, but they don't necessarily deserve praise, either. In other words, sometimes the best move is no move at all.
Often the fear of retaliation can work to your advantage. Say you're a seller, and someone has just purchased an item from you. The bidder pays in full, and you go ahead and reward the bidder with positive feedback. But when the bidder receives the item, he's not happy. Since you've already played your hand, the bidder then feels free to file negative feedback, or simply threatens to do so.
On the other hand, if you withhold feedback, the bidder will be much more likely to pursue a diplomatic solution to any problems that come up. Instead of leaving negative feedback, the bidder might politely request a refund, or, better yet, might even go away and not bother you at all.
For this reason, a wise seller will usually wait until the customer has left positive feedback, or at least wait for confirmation that the item has been received and the buyer is happy.
But does the bidder have anything to worry about? If an otherwise happy bidder leaves positive feedback for the seller, isn't there still risk of negative feedback from the seller?
In a word, no. Once a seller has shipped, the seller has everything he or she might've wanted. Unless the bidder does something grievously wrong, the seller has no reason to leave anything but positive feedback.
Not everybody retaliates. Some people never even leave feedback, negative or otherwise. If you're worried about retaliation, there's a pretty easy way to predict what any given user will do. Just go to the user's feedback profile page and click Feedback About Others (underneath the summary box) to view all feedback left by that user.
Here, you'll be able to easily tell how diligent someone is about leaving feedback, how prone she is to leaving negative feedback, and how likely she is to retaliate if a complaint is lodged against her.