Imagine that I wat to buy an account, someone makes a signed message, to prove it is his account, how do i prove if he sells, thatbI bought it? Because the address he signed with is still his.
You ask them to sign a message that the account was sold to you and belongs to you now.
Then make sure you stake an address you own for the account.
Most people don't want to know that the account is bought. Your recipe has a good chance to give that account a negative feedback.
I imagine that a honest seller would move his coins out and give you access to that address too. However, you have to trust the seller he will not use that address again (especially to sign again messages...).
This can be difficult as that address might be part of an account/wallet. If you export that one private key for the buyer in order to give him/her the key you are placing your whole wallet at risk.
Best is to ask the buyer for an address, then sign a message with your address [the one linked to the account] and state that account was sold and the new address tied to account is xyz. Buyer must then just go stake his address.
This is true, but if somebody makes accounts for sale, can think ahead and have a "simple" address for it. Else he will have, indeed, to "invalidate" the whole wallet.
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Since OP is Sr Member, I don't expect that he wants to buy an account, I expect that he wants to do his homework for selling it in the future. But it's just a hunch...