As a regular Ebay seller, I won't leave feedback until I know that the buyer is completely satisfied with his purchase. I think it's only fair.
In six years of selling on Ebay, only three items have been returned, due to my oversight when grading them. For two of these returns, the sellers left positive feedback (thanks Roger Stewart and A.N. Other!) - one didn't. The one that didn't leave feedback returned the record on quite unreasonable grounds, made one or two unreasonable demands in terms of postage, and I didn't leave the negative feedback I wanted to for fear of a retaliatory negative.
Dealing on Ebay is fun, but you'll always come up against one or two awkward bastards every so often.
I have a 100% positive feedback record, which has just hit the 900 mark, and intend to keep it that way. Since I like to know that the customer is pleased with their purchase, I think it's reasonable for the buyer to make the first move on leaving feedback.