Hi Annie - I have sold LOTS of book on half.com and love doing that. Textbooks don't move well though and the postage is high - even though your buyer pays that. The Sweet Valley books would probably move well at half.

Another thing you might consider is Bookcrossing which is what I now do with my books. Basically it is registering your books at this site, putting the number inside the front cover and then leaving them at public places to be "found" - it is fun!