[Mac Sirloin]

You just have to know.
They buy Pokemon games for more then anything else, its proven fact.
Call your local EB, ask for the prices and the following games.

Pokemon Saphire, Ruby, Fire Red, Leaf Green, Diamond and pearl.

Find these games at used game stores, they proabably cost less then what EB will give for them.

Shell out 50 Bucks to buy a bunch, then trade them in, you could make double what you spent.

Never try to sell something that costs less then 35$ there, you'd be wasting your time.