Can I really buy a Windows serial key from eBay?

Bruce updated his old Dell Inspiron to Windows 7 after buying a DVD on eBay for it. But after a month or so it stops working. So he bought another one with Windows 10 and it worked fine. Leo says that Bruce got lucky, most of the time, those serial keys are being resold over and over and often doesn't work with Microsoft's authentication servers. And with Windows 10, the serial number is assigned to the computer, fingerprinting it. Also, you don't have to buy the media, you can just download it from Microsoft directly. Once Microsoft authenticates it, you no longer need the serial number, and your computer will always be able to upgrade from here on out.