yeah, because if you leave them a negative, they will leave you one right back
Even if you payed for the auction 2 seconds after it ends... they hold it over your head until they get what they want. If you dont leave feedback, they will probably do the same to you, and just not leave it.
When I sell, I leave feedback as soon as I get the money.
When I buy, I leave feedback on how the seller handled the transaction. I dont care if they leave a negative on me anymore, because I can reply to it, and anyone who investagates the people they buy from, will easily see that I held up my end of the bargain, the other end didnt.