http://videogamepricecharts.com/Bascially, what this site does is it goes out to Amazon.com marketplace, eBay, and other sources and figures out the average price for a given game at this moment in time and plots that data over time so you can see if the price of that game has been rising or falling lately and the overall trends in the value of a game.
For example: say you wanted to get PaRappa the Rapper for PS1. Here's their entry for that game:
http://www.videogameprice...tation/parappa-the-rapper. Now, we can see that The average price of the game has been falling since December, with the average copy now going for $19.95, so now would be a great time to get it.
The only flaw I can see in this site is that it probably doesn't distinguish between CIB copies of games, however it seems like a great tool for guaging the overall second-hand market for a game. Just thought I'd share it with all of you, because I'm sure that someone here will like it.