I am not surprised as they can't seem to keep enough used in stock for Wii, additional the new price is less than the new for the other systems. Many are even less than $50 new. Hell they try and charge $29.99 for WiiPlay used with no controller.
Also some people enjoy pack-ins and gamestop doesn't handle those. I traded in DDR Hottest party and they threw the dance pad in the trash. Morons.
Also, while it is a ripoff to only get a $5 discount, they always have a percentage off special running.
Another think to take into account is we are still early in this generation so the volume of quality used games is not there yet, plus interest in used PS2 games and price they can get for them is probably falling.
Final* Word on Game Delays:
The game will not be any better or include more content then planned. Any commnets that say so are just PR hogwash to make you feel better for having to wait.
Delays are due to lack of proper resources, skill, or adequate planning by the developer.
Do be thankful that they have enough respect for you to delay the game and maintain its intended level of quality.
*naznatips is exempt
