I have Max Payne 3 but as retail so shall I ...?

Do what you want its your game after all. I think you can prob get stuff for it on steamtrades if you undervalue it by a bit.

Paste it for ninjas :P

eBay?

Sell it to omeone?

Could contact support to see if they'll allow a private giveaway for it, clearly indicating it's non-Steam. Minecraft and BF3 giveaways are allowed, so it's possible they make an exception, but who knows.

Otherwise, I'd say just try to trade it or find a friend to give it to. Don't put it up for ninjas.

Well, it's Xmas. Big chance you'll see some long-not-seen cousin, you can always give him a nice gift.

Or, like InsomniaNY suggested, try support about private giveaway.

So it can't activate one anything, not even Origin?

nah, neither of the max payne keys (i got one from turkish retail store and one from gamersgate) can be activated on anywhere but the official installer (i havent activated any of them) and i don't know if rockstar pass can be activated on steam or not, since i dont have max payne 3 on steam, i cannot check it. ill ask support and if it wont work out ill try steamtrades...