I've gotten a textbook or two through them.

Use the isbn number of the book you want at bookfinder.com will show other sites as well (make sure to set Canada as your destination). You can also use it to find the best price on textbook buyback, but that may not work as well for outside the US.

One tip when doing textbooks is definitely go by the ISBN if you want to make sure you get the correct one. Sometimes it is fine getting a different version number or an international edition, but not always. Also, when going used you aren't going to have codes for online homework access which depending on the course you may need. Often, I found it was cheaper to buy the course access + digital edition or loose-leaf binder edition of the text direct from the publisher than buying the book used+access or going through the schools bookstore.