Yeah, but imagine you jave some BTC in your wallet. One day you decide to sell them, just vexl them for a cash from someone.

You get cash and send them to someone else. Nothing will appear on your account to make the tax control suspicious you sold it and if so, the bitcoins are still in blockchain..

..the same if you use ATM or send them. They are still there and prooving they are under your control is nonsense. How this would be effectively done..