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A government cannot tax and regulate a foreign exchange, a few countries will regulate exchanges in their countries, maybe. But then the exchanges will just change their address to a nation that doesn't, problem solved

If you decide to sell some bitcoin through an exchange, that money will end up in your bankaccount. This transaction can surely be taxed if government passes a law that obligates the exchange to tax the transaction. Or if governments taxes the income (selling of bitcoin) coming from exchanges directly through your bank account. Perhaps a solution could be to de-centralize exchanges so that no nation's law is applicable.