Currently countries do not have protection for buying and selling with bitcoin. We can only agree to a 2-party transaction.
But we still have to follow the proof of purchase in this country
Pay taxes in the currency there.
I did not understand which country you are talking about. OP said here that two people in Portugal used Bitcoin as a means of transaction to buy and sell houses. That is, the amount that these two men charged for the purchase and sale of the house, they paid the amount through Bitcoin.