Transfer of ownership of a used computer to another party is completely legal. Typically, as the COA on the computer is "tied" to that computer, the original factory OS must also be transferred with the computer. Yes they will have full rights to use the OS, however the COA / OS are "tied" to the computer (motherboard) and cannot be used on any other computer system / motherboard.
If you have any further questions, please don't hesitate to ask.
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