The operators of Liberty Dollar tried to introduce a specie-based (silver) alternative to U.S. dollars. As you can read, the U.S. maintains that it has the exclusive franchise to any money (or in this case, tokens) circulating within its borders which can be used for value exchange.
Is barter trade/exchange illegal in the United States? Better leave those silverware ornaments in the cupboard - don't exchange them for someone's bicycle!!
If you exchange something of intrinsic value for something else of intrinsic value - is that not barter trade?
Does uniquely identifiable Bitcoin (digital goods) give the private key holder - the intellectual property right (intellectual property right value = fluctuating intrinsic value determined by market forces) to a world wide, efficient accounting network system - based on cryptocragphic key pairs, and backed by a secure network of thousands of computers (number and secureness determined by free market trading/exchange/barter forces), dedicated to this task?