Mji2001 :
It is an OEM. And its a reliable site. A few YouTubers (with over 100,000 subs) made videos about it
Many people use GTA for their game codes also, but those are grey market as well, that is not a fully legal key you are getting from them. It may be a valid MS key, maybe even not used, but if you ask how they get the keys, and check that against what is OK to use, it won't match up. That cost they are charging is about what the huge PC vendors like Dell would pay, or large volume licensees like corporations with hundreds/thousands of users. They are not valid for resale.
Going by YouTube is like picking a car based on what the kid at McDonalds drives, you need official sources.