If you have no set value in mind and just say "make me a *money* offer", then just make it a public auction. This makes it harder for any shadiness that is associated with the sale of any CC.
What shadiness? I've only ever had positive interactions buying and selling stuff on here, geekhackers strike me as honest and pleasant folk...
However, i do think we should get in the habit that beer people have of sending extra stuff with whatever we sell or trade as part of the tradition...in fact I'll start it now, buy this cc and you can expect another random cherry MX novelty key in the box with it...just feel inclined to pay it forward the next time you sell something.