Having laid the philosophical framework in previous posts, I want to restate one fundamentally problematic Libertarian argument.
The problematic argument starts with a common, relatively unobjectionable, moral intuition: if someone comes up to you, points a gun at your head, and takes your stuff (wallet, watch, car, or suchlike), you have been the victim of injustice. They follow with the assertion that the act was unjust precisely because you were deprived of your property by force without your consent. This reasoning isn't philosophically airtight (nothing really is), but most people would not argue the obvious opposite, that walking up to people and taking their property at gunpoint was just or even neutral. This step establishes a moral intuition (seemingly) about nothing but a set of objectively determinable facts about the world.
The second step is to argue that taxation fits this fact pattern. Taxation consists of the government essentially walking up to you with a gun and taking your stuff. Because it fits the same objective facts that we earlier agreed were unjust, taxation is therefore unjust. Anyone who disagrees is either mistaken or hypocritical. They may mistaken or ignorant about the objective facts, not realizing that that taxation entails taking property at gunpoint. They may be mistaken in their moral judgment, believing that taking people's property at gunpoint is just or neutral. Or, they may be hypocritical, judging one behavior at unjust and another just, even though the objective facts are identical. Taxation is just as bad as any other sort of coercive theft because the objective facts are identical. This step establishes consistency of application.
It's noteworthy that, unlike random robbery, the counter-argument that taxation is legitimate by social construction can be rebutted only by saying that the fact of social construction is irrelevant; any social construction that judges one case of coercion as unjust and another case, with identical objective facts, as just, is as mistaken or hypocritical as a single individual making the same judgment. Without this stipulation, the argument for the injustice of taxes falls apart, because the socially constructed legitimacy of taxation is not in doubt.
Up to here, although this Libertarian argument is still controvertible on other grounds, it remains consistent and coherent. We might have to be a little creative and broad about precisely which specific objective facts we attach our moral beliefs to, but we could create a political system that consistently and coherently applies the same political judgments of justice/injustice to the same relevant set of objective facts. It might or might not be a society we like or want, but it cannot be dismissed on the grounds of inconsistency or incoherence.
One type of Libertarian might just stop here. Coercion is bad, period, and should be employed as a necessary evil only to resist another's coercion. No mere social construction legitimatize any form of coercion except resistance.
However, the Libertarian who supports absentee property ownership is now in something of a bind. I define absentee ownership here as the situation where the objectively determinable direct use of physical coercion against the person of the owner is not required to deprive him or her of its objectively determinable use. For example, the occupant of a rented house is already in physical possession of the house; if the renter arbitrarily decides not to pay the rent, no objectively determinable coercion against the person of the owner is necessary. Indeed, it is the owner who must, in a objectively determinable sense, initiate coercion against the possessor to exert meaningful ownership.
One response is that coercion to enforce absentee ownership is socially constructed to be legitimate, even though the absentee owner does not possess the property. However, if social construction can legitimatize coercion to maintain absentee ownership, then social construction can legitimatize coercion to collect taxes. Remember, the argument against taxation above must be in some sense that because it is coercive, taxation is unjust regardless of any social constructions that legitimatize it.
Another response is that absentee ownership creates relevant, objectively determinable facts, such as titles, deeds, and leases*; the distinction between forcibly evicting someone from the house that they own is objectively different from forcibly evicting someone from the house that their landlord owns. But it is again difficult to distinguish absentee ownership from taxation: if absentee ownership can do so, then taxation can also create the same sorts of objectively determinable facts, such as statutes and regulations.
*accepting, arguendo, that titles, deeds, and leases can be classified as objectively determinable facts.
One very bad possibility is to simply declare that the objectively determinable facts magically change in an objectively indeterminate way when one approves of the coercion. Sadly, many Libertarians seem sufficiently philosophically naive to actually argue in a way that is impossible to interpret as anything but that the facts magically change. To paraphrase Jon during my recent debate, when the government is enforcing taxation, it is initiating coercion regardless of the social constructions that might legitimatize it, but when owners enforce their property rights, they are not initiating coercion.
The only other alternative is to simply say that yes, we can subjectively legitimatize differential coercion, applying different political judgments of justice and injustice not just on the objectively determinable facts, but also on socially constructed subjectively determinable facts, such as agreements, titles, deeds, leases, statutes, and regulations. There are, however, better and worse social constructions: the social constructions that legitimatize taxation are worse in some subjectively determinable way than the social constructions that legitimatize absentee ownership. Furthermore, we must determine the quality of social constructions by means other than looking at only the objectively determinable facts. (Alternatively we could allow that all social constructions create objectively determinable facts.)
The problem there is that I don't think the latter case can actually be made, which is why Libertarians don't often try to make it.