Barter is one of the easiest ways to make a libertarian argument.
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.
Now what if it's a straight quid pro quo: You do this and I'll do that. Is that taxable?
How about if it's quid pro quo with someone who's not my neighbor?
Extend the argument, and you start asking, "Hey, yeah, why do I pay income taxes? I'm trading my labor for their money. Why don't I get to claim my labor as an expense to offset my taxes?" And that's why all these tax protesters keep getting hauled into court by the IRS.