Ebay would be the best bet, in my view. Aside from Craigslist where you could probably net more versus eBay.
Check eBay and see what they (the codes) are selling for as a gauge. Then adjust your price lower by about 2 or 3 bucks compared to the most reputable seller and you'll get off of them pretty easily.
Right now, after Black Friday, I'm sure the market is flooded with unwanted codes so maybe waiting a week or so might do more good. That being said with X-Mas coming up if you take a bit of a loss and sell for 10 bucks less then the norm on eBay they would sell even quicker as a gift for someone to give.
I know Amazon sells digital codes. I'm not certain but I would assume third party sellers would be able to as well.
Or you could always make a thread on here in the trading forum. You might could trade the code (s) for a copy of Halo: MCC. That or barter with someone on a price but it being CAG you might not get as much as eBay.
I hope this helps and good luck.
