I don't know how you tell or care why a seller is legitimate, but it's very easy to find working windows 10 pro keys for a few dollars on http://www.allkeyshop.com/.
> I don't know how you tell or care why a seller is legitimate
Because illegitimate keys can be banned (and the hardware using them can be blacklisted) if they are found to be stolen or acquired in some other way that Microsoft does not like.
It's also a huge liability of you're doing any professional work with unlicensed and/or potentially illegal software licenses.
