It depends what you have. There's no real demand. There's some online places like
http://gametz.com/ where you can trade, etc. but there's little demand for PC games.
You can get some money at EBgames, etc. but they'll give you pennies, just like most of everything on their trade in list for console games unless they are very specific games or have a specific trade in value.
List what you have in the buy/sell of CP and see if anybody wants something. Unless it has multiplayer that is still popular, or it is an old classic game, it's almost worthless.