i wouldn't do it, looking to trade 4x MK8s into best buy is risky. trading online is just asking for too much trouble, shipping damage GC problems bad processing not to mention them banning you from trading for doing 5x copies of a game - the max is supposed to be 3 per platform-title as you prob know, if the trade in guy handling online decides to play detective you could get screwed.
there is definitely a market for the codes but as the window shrinks (must redeem by july 31st) it will drop in value. is it really worth all that effort to save like... $40? you could probably buy an extra copy or two of MK8 cn or redeemed codes for $10-20 on trade forums or ebay.
also imo its never worth "hoarding" console games, especially digital. Whats the point in having 10 Wii U games right NOW, its better to play a few, trade em off, and buy new ones. There will always, always, always be a bigger and better deal down the road. My steam library is 300+ unplayed games, and honestly I'll probably never play 99% of them but at least they don't take up any room. I don't hoard console games, play em and trade em and pass on any game I am not gonna play now, its gonna be cheaper down the road always. I only got 3 MK8 copies during the target $15 promo, because I doubt i'd play Wii party more than 5 minutes, just like I don't bother picking up all the crappy $5-10 Wii/Wii U games. When I game, I only want to play the very best
I completed an online trade without any issues for:
9x COD: Ghosts (PS3)
3x COD: Ghosts (X360)
3x MLB 14 (Vita)
All on the same order. The quantity allowed is up to 10 online, not 3. They allow more because they test each game out before giving you the giftcard. Use common sense when shipping. I doubt some guy is going to care whether or not you turn in 3 or 10 of the same game. He probably tests hundred each day...