Only Amazon (not the marketplace section) and Microsoft itself are authorized to sell Digital downloads.

This site is likely reselling MSDN licenses.

About MSDN or TechNet Product Keys
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.

In your case, what probably happened is, 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 need to purchase a genuine license, do so from the Microsoft Store:

http://www.microsoftstore.com/store/msusa/en_US...

I recommend this site, I purchased The Elder Scrolls Online: Summerset Standard Edition and I received the key within 4 minutes

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