a legit windows 10 pro license for 5$? That seems.. suspicious.
More than that, probably illegal. But with no harm falling on the enduser.
Those licenses more often than not arent even 're-salvaged', old oem license, but mass activation licenses MS hasnt disabled, and that are getting abused.
The issue for MS is the profit calculation here. They get more money off of their average user overall, by 'funneling' them through their legal ecosystem (stores (they get 30% off of every 'native' app you install through the app store), native ads, ecosystem lock in, advertising in general...). So what should they do with you?
If they 'disable' your license retroactively, they both get fallout from the non abuse license users in every block, and they are causing, people that usually arent even that tech savey (those who just wanted 'cheap') additional issues, that might have them switch over to Android, or 'iPad', where none of this is an issue.
MS pivoted their income model to 'service based' (as in not product based) a few years back (under Satya Nadella), and every day since then you as a user became more valuable to them while you were using their software - than you ever where, when they were still selling Windows.
(Calculation there goes: Almost no people back then 'bought' Windows either. Most of their customers got 'a new Windows', when they got a new PC - those also where mass licenses to OEMs, which were heavily discounted compared to end consumer prices. And this was before they sold advertising and 'apps' to you.)
So as a result as far as I know - they dont deactivate abused mass activation keys anymore. It just gets them bad press, and probably costs them money. Part of the calculation still is though, that you have a bad conscience.
And you should have one, because OEM licenses were a profit center for smaller Computer stores. Luckily they still have 'support'.
So dont do it, if you dont have to (legit OEM licenses arent that much more expensive), but if you are strapped for money, or really, really dont like MS... (Worst case scenario, you are down 5 USD, and can try again five times?
)
Now dont do that with other software licenses. I've literally seen ebay accounts selling 'activators' (basically cracks) as genuine office licenses f.e. Those are distributed 'for free' by their original creators, so don't be the sucker that pays for warez, because they wanted it cheap. Show some decency, be clever, not just cheap, and also pay for software, because most software houses arent Microsoft, cant make money on you by showing you OS level ads, or piggibacking for 30% off of other developers.
So either be 'good' all the way. Or choose your battles.
Dont become 'why should I pay?' guy.
That said - why should I pay more for Windows?
MS doesnt even seem to enforce any action against key reselling...
(Also, if they only have 'one windows version to support' for most of their customers, their cost structure, makes 'producing Windows' much less expensive. (Which is why they usually dont allow you anymore to turn off auto updating). And every new 'ad driven' scheme rolls out to a much much wider user base (because the non ad driven Windows (which they'd still had to support in the past) dies out faster), which they then can sell to advertisers as bigger numbers...
)