Libertarians of Weak Principles: Regarding Rand, so it was hypocritical for Karl Marx to buy vegetables at the market?
Yes, it was. On Marx's principles, he had a moral duty to find those whose surplus value was extracted and who were thus underpaid for their labor on the commodities he consumed, and compensate them.
You can sleep easy if you play by the rules even if you think the rules are non-optimal, as long as you point that out. That's Milton Friedman.
You cannot sleep easy if you play by the rules if you think the rules give you a license to steal. That's Robert Nozick, Robert Bork, and Ayn Rand.
That's the difference between utilitarian and deontological theories. Deontology is a bitch.