Speaking to IGN, Game Crazy's director of used games, Marc Mondhaschen, said the company did a study for "a very large vendor" and found that credit from trade-ins covered 20 percent of their sales over four weeks.
"So you got 20 points of their sales that wouldn't happen unless we had a trade business going," Mondhaschen said.
He also said that used games drive sales in DLC.
When 'The Lost and Damned' came out we started selling a whole lot more Grand Theft Auto IV, both on the new side and on the used side. Which, then, sort of funds people's ability to go play L&D again. And then the trade business funded all those L&D cards that showed up in stores.
It is one way to look at it, and as a consumer, I certainly enjoy the ability to sell games I've played, sometimes for substantial value. But the larger picture must also factor in those who do not buy new games at release, preferring to pick them up a month or so later. There must be a study that's isolated and quantified that consumer behavior, right?