All CoD4 CD-Keys around the world CANNOT be activated on Steam. Pr00f
So the best way is... to buy the game from steam store OR buy only key from the black market ( 90% of them are stolen or banned and that's why they are so cheaper, around $5-10 ).
I can offer you CoD 4 Retail Key
- 100% not used, legit, NOT banned, authorized, guarantee
( it's 100%, because it's brand new from store ). For $16
. But I know most people can't accept this, so noone will buy, and... that's all
Ofcourse can use MM.