abe books
powells
alibris
better world books
half.com

I've used all of the above, and others that I can't think of off the top of my head.

Sometimes it is hard with the used online sites to know what edition you are really getting. You can google the numbers of the ISBN if you have it.
He might want to check with the instructor to see if older editions are ok for the course.

I bought a piano book, listed in good usable condition, that turned out to be missing pages. The seller was apologetic and refunded payment and shipping, but it might be more hassle than a student would want to deal with--because he'd still have to find a usable copy of the book for the class.

Also, remember that Media Mail can be quite slow!