Maybe people wouldn't buy used games if the "new" ones were marked down every so often. There are certain games that are NEVER worth the cost of buying new. However, they might be worth 1/6th the price of a new game, and at that price point I'll buy.
Sure, there are certain games which go down a bit because they were popular, or because they aren't selling, but the markdowns don't entice me to buy.
After all, aren't ALL games used after the first day? I mean, Modern Warfare 1 isn't new anymore, right? Even if I buy a new DISC, it's still an OLD game. If I wanted to buy it earlier, I would have paid full price for it (I did!). If I didn't buy it then, why would I pay full price for it now?
Used games are simply filling a void in the market. People will pay what they think a game is worth, sometimes less, never more. If games weren't sold at fixed prices, the used market would die.
At the same time, the used market allows some game buyers to justify buying a game. If they know they can get a certain amount of money for their game, then the net cost of the game decreases. If there were no used game market, these buyers might think twice about buying those games in the first place.
Finally, don't let game retailers into the loop when it comes to used games. Sell on eBay or one of the many similar services. You are likely losing money just because you're too lazy to make the sale yourself, and that's just dumb.