I highly doubt that the keys actually come from the developers themselves. Usually these random keys are gotten from bundles or the sort. Unless you have any evidence to prove otherwise, I advise any potential buyer to use the same kind of consideration applied to other random Steam key sellers. I'll leave the following which I have said before and am tired of repeating:

To elaborate, buying from any random Steam key seller is always a bad idea. The way these people make money is that they obtain keys at low prices through various means. These means can vary depending on the seller. Some use keys they obtain for free or from bundles such as GOG or Humble Bundle-- these are usually the better (yet, still bad for the customer) dealers. Then there are also the gray-black market dealers, which use keys stolen from some developer or publisher database to make people believe they are getting a legitimate copy. The latter most likely results in the keys being revoked by Steam (yes, this is entirely possible but most people don't know it). Furthermore, while most sellers say you can get AAA games from their random keys, this is pretty much always a lie. All you will ever find in these keys are low quality or low cost games. I can guarantee you will never get a copy of GTA V or PUBG.