Let's say I help my neighbor paint his house. The next week he helps me build a deck on my house. Technically we just engaged in barter, and we both owe taxes on the work done. Why isn't this enforced? Because people clearly see that it's wrong. Not legal/illegal, but wrong.
I don't see anything wrong with taxing that transaction. I see a lot impractical, but nothing wrong. Taxes are applied to the exchange of value, money is simply a handy tool for keeping score.
Jay