I (personally) don't believe that used games have much of an impact on the sales of new games ...
The people who buy a new game and trade it in quickly to recover some of their investment are highly likely to use that money to buy another new game in the near future; and, although used game prices start off being close to the price of the game new, few people buy used games until the price drops to a level where there is a noticeable savings for buying a used game. Part of the reason why trading in games seems like such a rip-off (buying your game for often less than half what they sell it for) is because the game they buy today will typically be discounted several times before it is sold months down the road.
Now, I highly doubt that the person who is buying a used videogame at $30 or $20 would really translate into a new game sale at $60 or higher.
What I do think that the used game market demonstrates is that a lot of gamers do not place the value of a videogame at $60 and are trading in games or buying used games to make the price of a game more in line with their value level. If games were less expensive fewer people would see the value in trading them in, and more people would be willing to pay full price for the games. I could be wrong with the price level, but I suspect that if videogames were $20 few people would bother to rent, trade, or steal games and quite a few people would be willing to buy several times as many games; and overall, revenue levels for the industry on the whole could increase dramatically.
