Many of the cheap versions you’ll see on eBay and other website will be something called an OEM version. OEM stands for Original Equipment Manufacturer and is a term applied to companies that build PCs. Those desktops and laptops usually include a copy of Windows, so that when you walk out of the store with that shiny new Dell you can take it home and use it immediately.
While the majority of OEM versions end up preinstalled on PCs, it’s also possible to buy them as licence keys from places like eBay, Amazon or getmskeys.com. This is a common practice for users who like to build their own gaming PCs, or buy a second-hand device that has either no OS, or one that is out of date.