I don't know about the legality - but I think they're reselling unused non-retail keys, which probably skirts the Windows license agreement. You can get cheap Windows 10 keys. Like, dirt cheap.
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They're as low as a few Euro, not even $5 USD. From looking around and reading a couple of articles, you have pretty good odds of the key working. The desktop I'm using now came with a Windows 10 Pro (I think) key. I took a picture of it and gave it to someone here, but it didn't work for them - but I seem to recall they had other issues at the time. If you have a Windows 7 or 8 license key, that *may* still be able to be upgraded to
Windows 10.
In a couple of days, you'll have been at this for at least a year. As much as I don't like saying so, Linux may just not be for you.