You can use what ever form of currency you want. In order to promote liquidity and efficiency in trade the government backs only one form. The fiat money that we all use. If you choose to barter, and use other things and can find anyone to do this with, that is completely up to you and you face no "force". If you do want the federal government to back you up and enforce your debts and trades then yes, you must use the money which they endorse.