I buy used games almost exclusively. Occasionally EB games has those trade in 2-3 games of over $8 in value for a new one, which you can take advantage of. And lately, games valued that much are hard to find. You can check trade-in value of games here.
http://www.tivs.ca. Interestingly enough, Blockbuster often gives you the best value for trade-ins. There was also that thread in Buy/Sell here at CP where people were trading games.
In terms of straight buying a used game without trade-ins, you can find some pretty good gems at Cash Converters. A lot of those guys have no idea what a game may cost and may undercut it by a lot. Conversely, sometimes they sell shovelware for way too much. The other place I'll check is beyond.ca's marketplace. Sometimes you find good deals there as well.