You're right but usually there is a spread for any transaction and so the government could step back in and define a safe spread (something high like 20%) or th

You're right but usually there is a spread for any transaction and so the government could step back in and define a safe spread (something high like 20%) or the crazy (but efficient) option would be to allow the owner to define the spread. There would be a tax rate on the value and a different tax rate on the spread (possibly zero if the spread is set by the government)