IANAL, but there have been laws on the book prohibiting bartering in any substantial amount for a while. Of course, the IRS is not going to chase you down for swapping with your neighbor, but large scale "bartering" is definitely frowned upon. What's to stop people bartering a yacht for a house and avoid taxes?
You can barter all you want, you just have to pay the taxes on the fair market value.
https://www.legalmatch.com/law-library/article/bartering-law...
I don't believe that's true. The US dollar is only mandatory for two things: for settling debts (the lender must accept USD to extinguish debt) and for paying taxes. Otherwise people are free to trade as they please - euros, chickens, whatever.