The GameDays sale is for "new" stock only. Most of these items actually get sent to GameStop explicitly for the sale. Some games (like Katamari and Odin Sphere) are even reprinted so that they can go on sale.
You'll need to wait until GameStop does one of their occasional B2G1 free sales if you...
This kind of promo was done automatically by the Promo Wizard in the past. If there's no coupon code than this should be the case. However, the deal isn't shown on the "trade in values" page, so I'm not really sure what's going on.
Power trades or other similar deals never stack with other...