I was also going to suggest Bookscouter. I've had a fair amount of success with them because they go through so many buyers and you can see which ones are paying the most. I've never paid shipping as they do provide a label. Most of the buyers require that you have a certain amount to be able to sell. Typically $5 or $10 worth of books.
My daughter is starting her 3rd semester now. I'll have to fill her in on this when she decides to sell hers. Maybe she'll be better off than I was.
