Unknown Worlds has issued a warning to everyone not to buy Natural Selection 2 Steam keys from any shady websites. The Natural Selection 2 developers revealed that 1,341 keys had to be deactivated, as they were likely purchased with stolen credit cards. Naturally, many card holders asked for a "charge back" in order to be refunded. Not only does Unknown Worlds lose the $25 from the original sale, but they also get charged a fee of $22 for the process.
In total, the developers have lost approximately $30,000 from the entire debacle. They also mention that the only way to know that a key is legitimate is to buy it either directly from Steam or from the Humble Bundle store. There is also a list of authorized retailers, and it will continually be updated if more sellers become authorized.
Remember, stick to the websites that you know and trust!