We usually sell them to
Half Price Books. Of course, it always takes a half hour or so for them to finish going through them and tell us how much they're worth. By that time, we've already found more books we want. We usually just end up handing the money right back to them.

I've sold some books on ebay before - specifically my homesteading book collection. I made back much more money than I spent on them to begin with, but listing each one and sending them out was a hassle.
Oh, and I've sold books on half.com before, too. Still a hassle. I think I just hate having to ship things.