It has usually been buyers that 'forget' to leave feedback for me, but I have also been frustrated by sellers who wait for my pos. feedback before giving me any. I think immediate payment is worth immediate pos. feedback.
However, I feel that an even playing field for both buyer and seller would be a feedback system that would not release either parties feedback until both were recieved. This is such an easy thing for Ebay to do. By having a system that does not release feedback until the second person's has been entered, it would forced both parties to leave feedback if they want feedback. It would also illiminate the possiblity of retaliation and might make both people "think" before writing anything at all.
Before long, Ebay will take our right away to leave feedback for a buyer by giving it automatically upon payment, and the rest of us can only hope the buyer does leave feedback and that when he/she does, it is a true reflection of the item and service.