Most hardware etailers will sell it with a CPU, motherboard or graphics card or something, though they often don't count things like hard drives, USB sticks or peripherals such as keyboards, mice or monitors. Then again, some etailers will simply sell the license by itself, though they're not meant to, and Microsoft had a crack down on that a couple of year back.
The only restriction with OEM Windows is that the license shouldn't be moved between PCs. In the case of custom built PCs, a PC is defined as a motherboard. When the PC (or motherboard) is retired, then the Windows license dies with it and you'd need a new license. Replacements due to a parts failure (such as the motherboard) are deemed OK, so long as an entire PC upgrade isn't made at the same time. Retail Windows doesn't carry that restriction (though of course it can still only be installed on 1 PC at any one time, unless you buy the family pack).
Buying Windows licenses off of Ebay would be asking for trouble. I read yesterday that people are even selling Photoshop on Amazon UK, and it all looks fine... but only because the modified installation program patches a file to stop Photoshop from phoning home to Adobe. The DVD, box, instructions, shrinkwrapping - they're all done at home and look cosha. They're making a mint from it too - £250 as opposed to the £800 it would cost for the same license legally. It's not worth it.