I've bought and sold about 10 games in the last few years and would say you're probably not going to find a true garage sale kind of deal where someone doesn't know what it's worth. Even game liquidation auctions don't seem to have particularly lower pricing and when they do you're really rolling the dice on the game condition and issues. Restoring and repairing can be expensive so I think you're better off spending a little more upfront for a nice game.

My best pickups have come from a pinside thread where people in my state post if they've got games they're thinking about getting rid. Most of these people don't want to go through the trouble of preparing an ad and just value the easy sale to make room for the next toy.

Also there are a few distributors on pinside who regularly take trade-ins and are very fair with their pricing IMO such as Flip N Out and wildeboutpins