I always enjoy going here.
Since the books are nearly always used, they are nearly all inexpensive. The variety is wide, the store well organized.
You won't get far if you ask about a particular title, since they don't have every book in a computer. The fun is in browsing, and just seeing what you will find.
As some people have said, when you sell to them you may not get a ton of money. I don't expect to, since many of the books are not "current" and may be on the shelf for a long time. It's still better than a donation if you want a couple bucks. (Nothing against donations--I do that all the time at things like Goodwill, etc.)