New Windows 10 keys come with replacement Dell motherboards.
The Dell tech that does onsite warranty repair for us said he has a huge number of these already.
Guessing that anybody in the USA that is selling Windows 10 keys on eBay is actually selling these replacement keys.
That being said, you can also still upgrade to Windows 10 from 7/8/8.1 for free straight from MS.
https://www.microsoft.com/en-us/software-download/windows10
Also, if you want keys to be transferable, you should be logging into your computer with your Microsoft account.
When you do that instead of just a local account, that key gets put on your MS account and from what I have seen / personally experienced is that those keys are then transferable to whatever system you replace the old
one with.