it's higher than selling back to bookstore, but cheaper than used books in bookstore. Most of such deals don't involve negotiation, unless you want to. I've seen some Chinese fellow selling their used books at same or even higher than original price. Hehe, most people are not nuts... Another thing I recently saw is that we may want to photocopy a book if it's too expensive, but some Chinese people sell photocopies of the books, which is illegal and makes the image of Chinese people quite negative.
The above is just some tips from my experience of successfully selling 6 textbook packages in last two weeks.