If you think about it from the seller's perspective it's not in his/her interests to leave feedback before the buyer has left feedback in relation to the transaction. Unlike buyers, who can leave sellers positive, neutral or negative feedback, sellers are only allowed to leave buyers positive feedback. Therefore there is no point in a seller leaving a buyer positive feedback when the buyer could potentially leave him/her some very bad feedback in return if he/she isn't satisfied with the item purchased/won at auction and cannot be bothered trying to sort the matter out amicably if something does go wrong. The buyer may even try to scam the seller, but unless the seller has dealt with the buyer in the past he/she has no way of knowing whether the buyer is trustworthy or just out to scam him/her, so if you look at it from that perspective it's not really difficult to see why so many sellers hold off on deciding whether to leave feedback until after the buyer has left feedback first.
If, on the other hand, the buyer leaves the seller feedback first then if he/she leaves the seller positive feedback the buyer will often receive some feedback from the seller in return. It doesn't always work, but in many cases a seller will leave feedback for a buyer in the event that the buyer leaves some positive feedback relating to the transaction.
With regards to your point about a buyer paying within seconds of purchasing an item or winning an auction, many sellers would leave positive feedback for this type of buyer if the buyer's feedback for the seller was positive, as it would reflect an overall positive experience for both parties, not just the buyer.
