User Glowing Fish must be fairly sharp, as this "paradox" was discussed by none other than Robert Nozick in his seminal Anarchy, State, and Utopia. He quipped that an adequate theory of property or government should not depend on universal ownership of helicopters.
Any legal arrangement that permits one to imprison another by purchase of property does have a flaw. To claim that libertarianism cateogrically has such a flaw is to commit the straw man fallacy. Clearly the problem has been addressed in what's considered the foremost text on the subject. It's easy to pick a downmarket version of something interesting and set it up for critique, but one should generally presume that, if something is a major school of thought in academic philosophy, the facile rebuttals have already been thought up!
That also goes for welfare liberalism. The amount of literature devoted to the problem of moral hazard would surprise the average libertarian ideologue. Whether or not the answers are satisfying is a different question altogether.