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No, the holder/owner of the private key will not sell their private key or give it to someone else.
The ownership transfer will work through a 'transfer signature' that will be signed by the holder/owner of the address.
The message in the signature will contain the address of the new owner, but the new owner won't have the private key of the old owner's address.
but the new owner. does not own the original address AS YOU CLEARLY SAY
so here is a thing that you should learn. "not your key not your asset"
because you cannot ensure only the new owner has the key to the original item. means that buyer does not own the original item
you are not inventing a NFT system of owning the original item and transfering ownership of that item.. .. you are inventing an autograph system where people own autograph's of celebrity /historic item.. which then includes selling on rights for people to sign new their own autographs pretending they are part of the lineage of a autograph of celebrity /historic item.. but the autographs are not the original. nor signed by the original signer
its like if someone own an elvis autograph pretending it proves they are elvis. they are not sellling that autograph or selling elvis
they are letting the buyer write his own signatures that are not even any longer saying "elvis" in the scribbles
plus there is no way to limit who and how many you sell a autograph YOU create to of that original address. but you never sell the original address to stop yourself from rinse and repeating
thus VALUELESS