The Unknown Worlds website has a cautionary tale for those who may be tempted by the allure of discount Steam key resellers. Here's part of their explanation of what this is about:

Recently 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.