So do you leave feedback immediately when people pay you on eBay? In my experience (740+ ratings 100% mainly buying), I can't remember any sellers give me feedback first before I left feedback for them. And I've had plenty of sellers pester to me to give feedback for their item BEFORE they've left me feedback!!IMO, If you're selling exactly what you offer then you should leave feedback for the buyer immediately. They pay for the item they're part is done.
As for for the buyer leaving feedback, when the item is received.
I think feedback is the most important part of the transaction. I do a lot on EBay and if a sellers feedback is bad they're not getting my business. I plan to do business here as well so its important to see the feedback. It's the only way to know or trust the seller/buyer.