I don't think it's illegal to accept trade, but I do think it is illegal to refuse cash. It says right on the bill that the note is legal tender for all debts, public and private.
So if you where going to sell something to someone, I don't think you could tell them you won't take cash, only 10lbs of fish. Or something of the sort.
But I don't have any real qualifications on that. Just how I believe the system works.