Selling your game at Gamestop is a last-ditch desperate thing to do. They typically offer one forth of what they charge for the used game, so figure about $7 or $8 since they sell used Pokemon for $30. You could easily sell it on Craigs List for $20 or $25. Or get an account on Amazon.com as a seller and sell it there. The lowest Pearl price there is $20, Diamond is $22, and you get $4 for shipping that really costs only $1.
The best thing to go at Gamestop is to hang around the counter when kids come to sell their games. Offer them $1 more than the store will give them per game. That's how I got introduced to Advance Wars at a very low price.
If u see me comin better step aside. Lot of men didn't, lot of men died. Got 1 fist of iron, other of steel, if the right 1 don't get you then the left 1 will.