So I have bought a retail copy of Starcraft 2: Wings of Liberty. Now that I have it installed on my computer, and a free client available to download whenever from my battle.net account, should I trade it in? Or do I need the disc to install the client? Help is appreciated

You can't really trade PC games...

The game is tied to your account now. The key the game came with can't be used anymore, so it's completely useless to however might buy it.

Hmm. The Cex (Computer Exchange) store near me is trading them in for cash or credit.

You can't really return them either, as far as i know.

If you were to privately sell it, the game would be useless and someone would be very mad that the copy of Starcraft 2 they bought from a stranger doesn't work. Once you link the serial key to your battle.net account it becomes impossible to validate it with another account.

Seeing how the game needs an account just to play the single player, the person you were giving it to will only see the welcome screen.

As the game is tied to your account forever and ever, selling it or trading it would be a fraud.