Bitcoin profit

Suppose someone (early adopter) has 50 btc, he never declared it to tax authorities, and now wants to sell for profit. 50 btc is more than 5 mil usd now. How is possible to withdraw that money from broker to bank account? I think that the bank will call authorities asap and will block the transfer. I mean, we say an early adopter is a millionaire now but in practice how is possible for him to realize so much money?