What I do, is look around for old broken computers and laptops that have Windows 7 on them. As long as I either have the product key label, or can extract the windows 7 key from the OS, I can use it to activate Windows 10 for free. Once you have activated Windows 10 on a motherboard, it is always activated. You can pull the hard drive, install Linux, change your mind, pull that drive and swap it for a brand new drive, do a fresh install of Windows 10, and it will be activated using no key. Just click the "I DON'T HAVE A PRODUCT KEY" tab when installing and the OS will be activated when you go online. Simple as that. Don't waste money on sketchy eBay keys. If your Windows install someday goes to ---steaming pile of meadow muffin--- and you are using a media creation tool to reinstall everything fresh, then skip the key purchase, you don't need it.