As a seller, I usually don't leave feedback until I've heard back from the buyer either by email or via the feedback system, that they have the item and they're happy. For me, the transaction isn't completely finished until that point therefore I don't leave feedback earlier.
I do agree that paying immediately does mean a lot and in most cases when that happens the whole transaction goes smoothly. But there's always a possibility too that something could go wrong with shipping or perhaps the buyer doesn't like the item or has buyers remorse and isn't being reasonable. Whether it be the fault of the buyer or seller or neither, don't you think that the reaction from the buyer (and seller) in that situation should have a bearing on feedback too?
For example, I'm in Canada but most of my items are sold to people in the U.S. I always mail an item out within 48 hours after payment but sometimes it takes longer than it should to reach the buyer for various reasons out of my control. If a buyer is being unreasonable and rude or perhaps leaves negative feedback for something out of my control, shouldn't I be able to take that into account in my feedback for them? KWIM?
I hope this makes sense. I tend to get a little windy!