I was talking to a buddy of mine who has bought Windows 10 for incredibly low prices.
I thought those were either bogus or stolen keys, but I might have been wrong about that, because he's had no problem with it, and there are lots of vids on Youtube from system builders who have been doing that and having it work out fine.
He says the way to do it is go onto Ebay, search for "Windows 10 key", and buy an activation key from a place that is a certified trusted seller and/or has hundreds of reviews. You then get the install ISO from the Microsoft site, or make your own install media such as with Windows Media Creation Tool.
There are lots of folks on Youtube saying this works, like this (one among many):
https://youtu.be/5uVXSaziXJ4The next time I need Windows, I'm going to try it this way and see for myself.
IMPORTANT: I would make sure to buy in a way that they don't know your credit card. I'd use Paypal, for example. Just in case places selling these are doing it as a loss leader to get your credit card.