well... if you do trade it in... then what's to stop you from playing it anyway? You don't need the CD to play. If somebody else installs it with the same key then you prob both can't be online at same time....

I think that's what the guy at the store is referring to. he wants you to deactiviate your CD key so that you can no longer use it and someone else can.
Problem is I don't think that's possible.