The return policy is one of the kookier rules of buying used at GS. Basically a free rental service if you abuse it.
But I think the whole idea behind that is to get consumers in the GS "loop" where you turn in your old games and continue to buy used games from them.
$5 off is not worth buying used at all. For that little, I insist on eating the extra cost and giving the developer/publisher their due, not GS. Or better yet, just wait until the game goes on sale.