As a seller I post feedback on the buyer when his money clears then I ship the item. As a buyer I usually don't post feedback until I receive the item and have received feedback about my payment.

If a seller withholds feedback until I give him feedback he will never get it from me. Twice now I have paid promtly with any problem, but when I left positive feedback but added that I thought the shipping costs were high, they ( the seller ) left negative feedback for me.

If you don't say they are great and wonderful, they use it like blackmail.