This is what developers of Natural Selection 2 (UnknownWorlds) wrote:
Beware Shady Key Resellers and Discount Steam KeysRecently we asked Valve to deactivate 1,341 Steam keys that were purchased through our website. As a result, some Natural Selection 2 players who were previously able to play the game on Steam are now receiving a “Buy Again” option instead of the “Play” option. This episode is a prime example of why all gamers need to beware discount steam keys and those offering them. We deactivated these keys because they keys were purchased with credit cards where the card-holder initiated a “charge-back.” A charge-back is a consumer protection mechanism offered by payment companies such as Visa, allowing a card-holder to dispute a charge on their credit card statement. This means we never received payment for the game. In fact, we were charged a fee by the card issuer for the charge-back. For these 1,341 keys, these fees totalled around $30,000.
Right now there are only two official places to buy Natural Selection 2; through Steam or through our website via the Humble Store (The Humble Store option is temporarily disabled due to this issue). If you see Natural Selection 2 available anywhere else — like the many sites out there that sell Steam keys at a discount — then you are not buying it from us and there is no way to know if that key is legitimate. As as result, we strongly discourage purchasing from these sites.
If we offer Natural Selection 2 from other stores in future, we will make a news post informing you of the legitimacy of that store. You can always find an up to date list of authorised stores right here.
We don’t know how exactly these sites obtain their Steam keys. It seems likely that they were originally obtained from our store using stolen credit card information. Keys were then sold through a handful of questionable sites to people using legitimate credit cards. The owner of the stolen credit card ultimately disputed the charge and we lost the sale. In total, we lose ~$45 per transaction of this kind, due to the charge-back fee (~$22 fee + $25 game price). Meanwhile, the unauthorized key reseller kept the money from the player who ultimately received the bad key.
If your key was deactivated, we recommend you contact the site you purchased it from for a full refund. It’s unfortunate that players who believed they were buying legitimate games were hurt by these unscrupulous resellers.