If you have the patience, sell your games on eBay. You get real cash and more value than those credit shops will ever offer you.

I also found a loophole with stores like EB Games or Gamestop. Sometimes they offer you trade in deals towards pre-orders where they will offer you extra credit, more than usual, or something like 50% extra credit for example. Let me explain...

You trade in some games and your store credit gets thrown onto a plastic swipe card. When you want to buy games afterwards the store clerk swipes your card and the credit is applied to your purchase. Now if you trade in some games and receive bonus credit towards a particular pre-order, the credit is thrown onto this plastic swipe card and remains locked in there. Call the store up later and cancel your pre-order verbally. The store clerk does not have access to your swipe card and cannot remove any store credit that was applied as part of the pre-order promotion. Now keep the bonus credits you received and apply them to a game you actually want.