First, I realize that non-state actors can be coercive and violent and the fact that there are such non-state actors does not make the state any less dangerous, although it might provide one argument for why we need a state, a point which I suspect libertarians recognize.
Second, I do not wish to deny the extent to which the state is implicated in coercive actions by putatively voluntary associations. The various manifestations of the Ku Klux Klan are an obvious example of violence being carried out supposedly independently of the state but existing largely because the state suffers them to exist. (And in some cases, I suspect, local state actors--members of the local police or sheriff's department, for instance--might have participated, further blurring the distinction between state and non-state actors.) The example I cited in my post below was less extreme, and the decision of the Canadian federal government to go after the coal "combine" through an intimidating hearing and, later, through a federal antitrust law suggests that the Canadian state did not, at least not officially, tolerate such an action.
Third, I think my conceptual difficulty with libertarianism is that I am not clear what libertarians are against. I know they are for "liberty," but it is unclear to me that they necessarily oppose non-state impediments to liberty, or at least they don't do so as libertarians. This is not a knock against libertarians, just a qualm I have with libertarianism, as I understand it. And of course, some libertarians, such as David Bernstein at the Volokh Conspiracy, even support some positive action by government to redress certain infringements on liberty by non-state actors. I have in mind his essay on a libertarian approach to anti-discrimination laws. Click here to read it. Particularly, I'm interested in the following assertion:
Consistent with longstanding classical liberal suspicion of monopolies, many libertarians would allow the government to ban discrimination by such entities.In the context of anti-discrimination laws, such as the Civil Rights Act of 1964, this suggests moderate libertarian support for such laws. (I should note that Bernstein does not claim that his is the only libertarian-oriented position on the subject. See his post on the Volokh Conspriacy here for links to different takes on the same issue.)
Fourth and finally, I think my principal conceptual qualm with libertarianism, as I understand it, is that any action in which a "public" is affected is almost necessarily regulated somehow. (By "public" I mean what John Dewey meant in The Public and Its Problems: a person or group of people affected by the actions of others.) Any "free market" and any social interaction has a set of rules and deviations from those rules have consequences. The substance and contours of these rules may be defined by the state, or by convention, or by some supposedly non-state or quasi-state entity like a "board of trade." There are better rules and worse rules, and I suppose an argument is to be made that a state, arrogating to itself all the "legitimate coercion" under its jurisdiction (although in the final analysis, I'm not sure states actually do this, or that they don't do this without facing constant contestation if there is a strong civil society), might not always or necessarily be the best instrument for setting up these rules.
Again, I fear my post on the matter has become a bit incoherent. My "clarifications" just raise new questions about what I really mean here. (I'm sure glad I post these things pseudonymously!)