Be careful with these kind of sites.
Valve deactivate over a thousand Natural Selection 2 Steam keys sold in $30,000 credit card scam
Over a thousand Natural Selection 2 players have found their copies of the game deactivated on Steam after a scam that cost developers Unknown Worlds Entertainment $30,000. The copies were purchased through third-party sellers, who are believed to have bought the keys in bulk using stolen credit card information, before revoking the payments.
http://www.pcgamesn.com/valve-deactivate-over-thousand-natural-selection-2-steam-keys-sold-30000-credit-card-scam