When I was trying to sell my old textbooks at the end of grad school, I went on all of the websites and put in the ISBN number of each textbook. The site will then tell you whether or not they are accepting that particular edition of that particular textbook. I did this for every single textbook on each of those textbook buyback sites, because each site may give you a slightly different buyback price, and some sites will take books that other sites will not take.
Once I figured out which sites were taking which books for the highest prices, I printed out the free shipping label and sent the appropriate books to the appropriate sites, then waited to collect the cheque. It was a lot of work, but I think I got back at least 50% (if not more) of what I paid for the textbooks.