Why wouldn't you be able to sell your Gamestop Game somewhere else? I suspect what is happening is that Gamestop guarantees to buy your game for a certain price, which would be great (as in being a real plus compared to ebay). For another beneficial thing about used games is this: maybe 60$ is too much for me for a game, but I know I can sell it for 20$, so it really costs me only 40$. Therefore used games are good for the industry - without used games I might not buy any games at all. With the guaranteed buyback price of GameStop, I know in advance how much a game costs me. Some people might prefer that to putting in the work to sell on ebay/Dawdle for an uncertain price.
As for the original case, it makes no difference who buys the used game, what people seem to care about is the amount of money spent for buying new games. And that is the same for GameStop and for Dawdle.
Right - you're making our point. When GS buys a used game for $20, they re-sell it for $40 (49% margins). That means the value of the used product is $40. If you can sell it for $40 on Dawdle, a new game only really costs you $20.
That means your $100 in purchasing power gets you five new games: the publishers, developers, and retailers only benefit when you buy new product. We're trying to show them how used games can work in their favor if you cut out the GameStop middleman.
Edit: as for the uncertain price, we're working on it. With our StandingOffers, we might be able to find people willing to guarantee you $30 or $35 up front.
You should think about the magic net gain. You can not just buy games for 40$, there must be buyers of used games. These don't show up in your equations. There is only so much money people will spend for games, you can not create more.
I am not saying that Dawdle might not be a better deal for the end consumer. Certainly lower margins means cheaper games means more games sold. But I don't think GameStop are crooks, they simply have an alternative business model. They might be more expensive, but in exchange they offer a different service (guaranteed prices, no hassle with online selling of your game). I don't know GameStop or Dawdle, btw.
As for the original case, it makes no difference who buys the used game, what people seem to care about is the amount of money spent for buying new games. And that is the same for GameStop and for Dawdle.