Gamefly discounts a selection of used games for PlayStation 3 and Xbox 360 with prices starting at $5.99. Plus, all orders bag free shipping. A couple best bets, with comparisons to new copies elsewhere:
Amazon Warehouse Deals offers Prime members the used Skylanders SWAP Force: Jade Fire Kraken Figure for $3.55 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's $6 below the best price we could find for a new one. It comes in the original packaging and includes a trading card and sticker sheet.