The Best Buys around here sell used games They're more than they would be at a yard sale, but most are decently priced (and they come with a return policy if they're defective). They also have a bin of cheap games, like $4.99 or less. Most aren't great, but you can find some decent ones in there if you're willing to dig.
I also buy from SecondSpin.com. Their regular prices are okay, but they frequently run good sales/shipping deals, just check the banner at the top of their page to find the current deal and the necessary code. They sell used games and they guarantee them to work.
And +1 to Gamestop and Goodwill. Also, Salvation Army stores. Target will also sometimes run good sales like buy 2 get one free (which can come in handy if you're looking for pricier/newer titles), or gift cards with purchase.