When a seller recieves a prompt payment for an auction, they should leave positive feedback at that time. Why wait until the buyer posts their feedback? Leverage? If you've sold your product truthfully and provided good service, there's very little chance of you recieving bad feedback.
My ONE bad feedback was from a seller who sold me a very, very bad fake Gucci wallet. I mean, it was horrible. I sent her scads of emails requesting a refund. No reply. So, I posted negative feedback. She immediately retaliated with bad feedback, even though I immediately paid for the wallet and patiently waited 2 weeks for the bum wallet to arrive with no compliants.
Bottom line: unethical sellers like to withhold feedback as a means to fend off their own negative feedback.