The Lounge is rated Safe For Work. If you're about to post something inappropriate for a shared office environment, then don't post it. No ads, no abuse, and no programming questions. Trolling, (political, climate, religious or whatever) will result in your account being removed.
I never paid more than 50€ for windows products. They are usually reselled unused licences from an enterprise licence pool. As an example, I am the IT guy from the local school in my spare time, which we can see as a little company (50 laptops, 1 server, 2 xerox, ...) and we buy MS Office licences for new computers in the school. The microsoft volume licensing scheme delivers in our case usually up to 5 more licences than the bought amount with each purchase, that you may or may not use - do not ask me why. I won't go into details but due to public money and regulations and bla bla we HAVE to buy new licenses and cannot use these free ones. I could easily resell the unused licence keys because I will never be able to use it, and that is precisely what companies like the one you mentioned do.
I do not think it is entirely fully legal, but I do not think it is written anywhere that is not, so ...
Long ago, and with a different version, I had some trouble with a MS Windows license. As it turned out, I had a product that was validly installed via an OEM key that had since expired. It was later disabled by MS because it was being used 'fraudulently' . Also, it could only be used on OEM versions and OEM versions could only use OEM keys.
In my case, I had a valid MSDN key to reactivate it but it was rejected. There were all sorts of unpleasant problems, but only after it went bad and I needed to reactivate it.
".45 ACP - because shooting twice is just silly" - JSOP, 2010 ----- You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010 ----- When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013