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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 not even inventing an autograph system where people own autograph's of celebrity /historic item..
the autographs are not the original. nor signed by the original signer
its like if someone owns an elvis autograph including the pen that signed it.. pretending it proves they are elvis.
but they are not selling that autograph or selling elvis's pen
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 created
thus VALUELESS
what you are creating is a "chain love letter" system
elvis wrote in 1969 "i love casinos" signed ELVIS
casinos wrote in 2022 "i love onesigcasinos" signed casons
the signature or ability to sign onesig now has.. has no value. as it has nothing controlling/ownership of anything elvis