I always leave feedback for buyers when they pay, and all my buyers have paid within minutes. The sale's obviously not complete at that point, but it's my way of saying "you've held up your end of the deal." I also e-mail buyers when I get payment and let them know that I have a return policy...I don't state that on the auction page because I don't want it to be abused. I want them to know (without saying it of course) that if they aren't happy, they should contact me before they think about leaving neg feedback.
I bought a $500.00+ item recently and emailed the seller to let him know I received it (and hoped he would leave feedback w/out being reminded since I paid in 3 minutes and am the easiest buyer to deal with.) First of all, it took him 4 days to email me and to ship the item after I paid, and he only had the one auction at the time. If I had an auction ending for any amount, i'd be around to service the buyer. I got a response saying that he was glad to know it got to me and to not forget to leave positive feedback..HA HA NO WAY...JERK!