The Book Store is starting their 2nd semester textbook buy-back on April 14th!! This is a great way to lighten your load and also make some extra cash right before the summer... They usually take over 50% of the value for themselves, which is quite a bit, but the benefit is that you get your money right away--no waiting around hoping your book will sell, etc. It's the best no-hassle way to sell your books fast.
... that said, the other option is to sell it back to the used book store, and then wait for your cheque, which is usually considerably more money, but you might have to wait a while (andthey keep some of the selling profits as well).
If you want to get the most value, and you don't mind waiting around a bit or putting a little extra effort in... check out amazon.ca Marketplace (you set whatever cost you want and get reimbursed for shipping -- you'll make WAY more than selling it on campus through our stores). OR, try listing it on Kijiji.ca > London, Ontario and selling it to a local, OR, post it on Facebook's marketplace, or even posting some posters around campus if you have some really pricey books you want to get rid of!
Good luck to everyone!! (and if you need any MIT textbooks --> Get in touch!! haha)