Unfortunately, if the new generation of Silk Road type platform proves effective and resilient, I think that any large scale crime syndicate or trafficker with the wherewithal to pay is going to start running their own.
And granted, it may just be an arbitrary level of abstraction to say the state is necessarily any more just in generating or enforcing laws than a third-party entity, but I do believe that in the case of decentralized black markets, something other than the markets themselves are going to need to act in that stead, because there would otherwise be a conflict of interest in trying to rein in illegal activity in communities whose entire purpose is profiting from illegal activity.
Though I suppose there might be precedent, for the sake of the self-preservation of the whole, for an Anonymous-like group to step in and keep the real scum at bay.
Which might bring everything back full circle, either the Feds get called or vigilantes.
No, you should be frightened by the fact that governments, being monopolies, tend to create less economically efficient products (including laws) than competitive markets, so if you assume that societies are better off if they have legal systems which prevent slavery/kidnapping/etc., competitive markets will be more likely to produce those legal systems than governments.