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.
If you know anyone who has ever worked at microsoft they can get you a copy with a MASSIVE discount. Living in Seattle I just buy all my microsoft products through friends.
Elite controller was $99, and I laughed when it was on sale for like $150 last week as a best ever deal.
If you only need a single copy you are best suited buying from Microsoft directly.
If you need multiple copies you can work with a VAR (Value Added Reseller) to get better pricing.
CDW.com is the big name in VAR's but they have enormous turnover in staff. I try to build relationships with smaller VAR's. I like these guys: https://greenbeetech.com/ it's a small company with two guys running it who have decades of VAR experience (former CDW people).
Navigating the process of compliantly licensing Microsoft as you scale up is quite a job. Having a good partner to help your organization stay compliant is very valuable.