most game sites have a way to check your code. I know all xbox and microsoft codes you can enter in and it will let you know what they are for and if they are valid before you hit redeem.
I sold about $1k worth of xbox live codes on G2A, so you can start there.
Only thing is, G2A has a HORRIBLE payout system.
If you sell $10 worth and owe them a $1 listing fee, you are supposed to get $9 after 14 days.
However, if on day 13 you sell $100 more stuff, they take THAT listing fee out of what they were going to pay you.
It's confusing but the more you sell on G2A the longer you have to wait to get paid..it is really dumb.
I sold on ebay for a while too, but just be careful. You WILL get scammed, so you have to have high enough profit margins to cover that because paypal will never side with you.
Best bet is to sell with G2A or on your own site right now.