In Buy/Sell/Trade

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I've never tried that personally, but I know some companies will only have so many copies of one game in their store. They may only take so many. I've never really dealt with them on that end, so I'm not sure what kind of policies they have.

You give them 5 copies of one game and get 10$ off every new game you buy from them? (Did I get that right?)
Then it depends on their prices whether it's a good offer or not...