This is something I've been aware of for a few months, only paying attention to it when it appeared in a Paid announcement on DealNews, who provides *at least minimal* vetting. There's a somewhat well known company selling win10 Pro keys for $15. I bought one, & got the key in a couple of minutes -- I stayed logged in after paying with PayPal, & as soon as the money was transferred, a button appeared to get the key.
I then plugged the key into one of my copies of win10 Pro that had been licensed as an upgrade. My Microsoft account didn't show anything new, but it appeared in Control Panel -> System, & when I used NirSoft's ProduKey.
The reason I got the key was if someday in the future I upgrade this PC's hardware, e.g. new motherboard & CPU, I expect any upgrade licenses for 10 Pro will deactivate, & I won't be able to re-activate them, though Microsoft could change their policy by then [10 might even be free for all I know]. If I buy an OEM key from a retailer like Amazon or Newegg, it should be possible to re-activate in that circumstance, but there's no reason to pay $110 or more until I have to.
This new key I bought may or may not allow me to reactivate if/when needed -- it's a $15 gamble. It may come with the same restrictions as the OEM copy that comes with hardware, which is more restrictive than the OEM builders license I can buy at Amazon or Newegg. And/or if I try to re-activate I might wind up with the same experience WR had with cheap win7 keys he bought, with Microsoft saying I should never have had this key to begin with. At any rate, I took a $15 gamble in hopes of saving $110 or so later on.
scdkey[.]com/microsoft-windows-10-pro-oem-cd-key-global_1227-20.html