But would someone ever buy bitcoins from someone without actually permitting a transfer? How would you ever have an assurance that seller has relinquished contr

But would someone ever buy bitcoins from someone without actually permitting a transfer? How would you ever have an assurance that seller has relinquished control of the coins? Example: I own one coin. I sell my private key to you for $5k fiat. You take my private key and sit on it thinking it’s yours. After a use a copy of the private key to transfer the coin to a new wallet I own. I’ve now stollen the coin I supposedly “sold” to you. What am I missing?