Like those folks ^up there^ said, feedback is optional. When I was selling off some old books on eBay, sometimes buyers didn’t leave feedback for me, either. It’s like, “Oh well, move on.”
There are times when the buyer/seller isn’t necessarily the best, such as when they take a few days to get around to paying, or don’t provide details (like where to mail the thing) promptly, send too much email or not enough, or take a while to get to the mailbox with what I bought. These are weird-feeling situations. If it were an obviously bad situation (didn’t pay, didn’t send item) one can leave bad feedback, but there are times when the item or payment still made it and you can’t really say anything bad, can’t really say anything good. Sometimes the kindest thing to do is say nothing at all.
And then, like @funkdaddy said, sellers are busy people too. Sometimes I’d sit down and do all my feedback at the end of a week or something, rather than one at a time.
Yes, people probably should leave feedback all the time, just like we should call our Moms now and then and donate to the hungry. :) But sometimes, people just don’t. I don’t know why. But as you buy and sell more, you will have other chances for feedback. It’ll be OK.