I am sure that there are lots of disreputable eBay and Amazon vendors. Somehow, I seem to have been lucky so far. I have been using eBay for approximately 20 years and was ripped off for about $20 just once. So I am fairly good at picking reputable sellers. But I can only speak of the actual experiences that I have had. I was as skeptical as anyone here when I purchased my first product activation key from eBay, and totally expected it to be some type of scam. Maybe they still are, but I do not know what it is. Should all the licenses from old computers just be lost when they are sold? That might be the way that they are written... I have never tried to wade through all the legaleez. Maybe someone here can tell us.
There used to be all sorts of key generators available which provided keys for Windows XP that lasted for a long while. That got tightened up pretty well when Windows 7 came along. But then it was found that the OEM keys license keys could be copied with a utility and then reinstalled into a clean install to get rid of all the crapware that comes with new computers these days. But it is very helpful to have a legitimate licenses that can be used in virtual machines especially with Windows 10 since it “calls home” basically every time the OS is connected to the internet.
I have heard that many surplus Windows 10 licenses come from creative souls who “recycled” thousands of corporate computers and then did the free upgrade from Windows 7 to Windows 10. I do not know if this is accurate or not.
I bought a few Win 10 keys from ebay vendors in UK. About $4 each. They have all worked so far. My speculation is that Microsoft gave out Win 10 keys in UK (Europe too) like candy in order to keep/expand Windows and Microsoft market share. That Apple has them running scared.
Go to ebay UK to look and pick a vendor with a good track record. Obviously the Win 10 key comes via email and you use it with Win 10 downloaded direct from Microsoft.