Technically speaking, bringing it to a store for credit is what we call "illegal" and will not be allowed to be discussed here. Ebay it or put it on Amazon Marketplace. Or, if you want to be a good CAG member, offer it up on here at a nice price. There's a reason we have a trading forum.
Most Ebgames don't care if the system or game has been opened. They get $85 in credit for a SLIM model PS2 right now. Otherwise Ebay will net you $115-$125!!