"Artificial" scarcity, even of so-called necessities, is not indirect coercion so long as it's the choice of the property owner. No one is entitled to the provisions or labor of another because of lack or need. If someone decides it's not in his interest to distribute the fruits of his labor, that's his right.

And yes, people can walk away. Human beings have being walking away for tens of thousands of years. You don't need to read libertarian literature to know the phrase "vote with your feet".

A government with a monopoly on force and enforcement of contracts is as much a feature of a libertarian society as it is in a contemporary one, and so the comparison involving the USDA is germane. The contention is one of degree and nature involvement. A private property owner in a libertarian society must still follow the law to the extent that such laws don't violate his rights.