http://www.onlinekeystore.com/home.php I bought black ops $20 cheaper Some good deals elsewhere on the site
I wouldn't trust any CoD keys, Activision are known for disabling any keys that aren't related to that region.
I'm not saying it doesn't, just with MW2 from what I heard, Activision literally went around and removed the game from peoples accounts (or at least got Valve to do it).
They just scan in the cd-keys from mass-bought/defect packages/sometimes stolen boxes from countries where they're much cheaper.
It's not so much as illegal / scam, I use a website similar (g2play.net). Like I said, CoD keys are just a very touchy subject with Activision who'd rather you paid full price.