Avoid them...
They are genuine Microsoft product keys, they are actually retail licenses, but it is intended for a particular product channel either the Microsoft Software Developer Network (MSDN) or TechNet for IT Professionals who pay a subscription fee. The main purpose is for evaluation purposes. The great thing about them, unlike trial Microsoft software, MSDN or TechNet keys don't expire.
Because the agreement under which the subscription is provided is a single license, none of the software should be distributed outside of it. Even though its $50,000 worth of licenses, it is for one person only to use and no one else. Unfortunately, regardless of the licensing terms, persons still abuse the program, either giveaway product keys or resell it on auction sites. Microsoft licenses it in good faith that customers won't do so, but I guess human nature wins out.
If you bought an MSDN licensed key, which carries up to 10 activations unlike full packaged retail licenses which only carry 1 activation, the person who sold it to you probably sold it to 10 other persons. Somewhere along the way, one of those persons might have installed it on a second system, activated it, because it went past 10 activation threshold, Microsoft detected it that it was being abused and blocked the key from further use.
If you want to be guaranteed a genuine license, purchase it through the Store or Amazon.com (not the market place section).
https://products.office.com/en-us/get-started-w...
An immediate sign its not genuine is if they are selling the Pro Plus edition, which is only available to large organizations.
