For the individual user, the risks of buying Windows on eBay are primarily functional rather than legal.

: Since the product is just a string of 25 characters, it is incredibly easy for a seller to provide a fake or already-used code. While eBay's Money Back Guarantee provides some protection, the low price point often makes the hassle of a refund feel not worth the effort.

: Because many of these are volume keys, Microsoft may eventually detect that a single "business" key is being used by hundreds of individual eBay buyers and blacklist it, resulting in your Windows suddenly becoming "unactivated."

: These are intended for large manufacturers like Dell or HP to pre-install on new hardware.