GMG trade-in question

Hey guys. I was wondering how to trade games in on GMG? I see the tab, and it says

"Your library is empty.

Start adding games now or import them from playfire!"

How do I do this, and is it worth it? I really want to get The Witcher 2, and it is 20 bucks everywhere, haha. Was thinking this might be a good way to nab it without spending too much.

Just go to library then games and if its eligable for trade in you will see the option ( at least last time i traded it was like that )

BTW only certain games can be traded and witcher 2 i think it cant

I want money to spend ON the witcher 2. I am not looking to trade it in, I am looking to buy it. Oh well, thanks for the help! Kicking myself for not buying it at the last steam sale.

Well its 15 when you use greenmangamings 25% off coupon. Not a bad price at all