Is it fiat currency?The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
I know it's not just the $US you make as you have to report foreign currency as well.
Say someone actively barters for a year - never exchanges his goods or services for money (perhaps a successful Timebankee) but still manages to acquire an increase in net value every year. How would tax law apply to them?
What if you are a gold digger, but never sell your gold - as you know people who will accept gold as a medium of exchange, do you report this as income?