The feedback should be for the entire transaction, or at least the most important part of the transaction.

If every seller gave automatic positive feedback once he is paid, then the feedback system would be worthless -- it wouldn't give an accurate picture of the conduct of the buyers. It is most important that the buyer not be an unreasonable pain in the ass about the items he receives. Maybe it would be wise from the individual seller's point of view to automatically give positive feedback when he is paid -- it butters up the buyer, but --again-- it does not give an accurate profile as to what type of buyers you have.

Since the sellers can't give negative feedback anymore, it almost doesn't matter what the seller does and when.

Robert