Giving someone your seed phrase is inherently insecure since you could have just copied it. To make sure the buyer is the only one in possession of the private key, he MUST move it to a new wallet.
Unless you have a multi-signature setup where the parties' don't know each other and transfer the keys to the new parties.
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Oh good point, didn't think of that actually! I'm stumped then.
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This is why OpenDimes were invented. It's the closest thing there is to "physical bitcoin". It let's you verify that nobody has the private key and allows you to get the private key if you ever need to move it on-chain
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