I've had really good luck buying on half.com.
It seems as though every book I've gotten was about 1/3 to 2/3 of retail even after shipping, and the books have always been at least as good as advertised. There's another site which is used bookstores, but I can never remember whether it is bookfinder.com or bookfinders.com. Whichever it is, the other URL is an "adult material" site. Bookfinder prices aren't as good, but frequently the selection is even better.