I would send an email just to make sure they are satisfied, and in the email include a link so they can easily click to leave you feedback.
I say something like, "I have left you positive feedback for this auction. If you feel it is deserved, please consider leaving me a positive feedback by clicking here ...(link)... . "
I have quit buying from sellers that don't leave feedback, and use the "ask seller a question" to let them know why. I once bought from a seller who had 7000+ feedback, and only would leave feedback for his buyers if it was a negative. I didn't check ahead of time and he never left me a feedback. Now I check in advance, and make sure I tell them why I wouldn't bid on their auctions. If you have time to leave negs, you have time to leave positives too.