Since You are Newbie i will just post this in case for other people.
Don't buy a CD-Key from random person!
Here i explain You why:
There is an option in STEAM to recovery account just by typing CD-Key and email. If Someone have this 2 information He is able to send a ticket to steam and reset STEAM account with new log - in details.
This is quiet new method by scammers going all around last days.
I don't say that OP is trying to scam or something but please use option to send CD KEY directly to YOUR EMAIL not getting CD KEY from someone like this. and btw DON'T BUY STEAM GIFTS as they can be REFUNDED in few weeks. Which means You got a game for few weeks from seller and he took it back.
Anyway GL Sir.
Use G2A for games
