Those transactions are still going to have "meatspace" components which are, and will remain, the weak point that law enforcement will target.
Tor and bitcoin already exist, if mobs want to use those systems, they already can. The utility in systems like SR is reputation management and connection discovery. In other words, this system would really be nothing more than a craigslist that doesn't claim to take down ads for hookers. It shouldn't frighten you.
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.
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.
And there also is a number of unhealthy uses of cars, like using it to transport yourself right after robbing a bank. Should we ban cars?
Anything that can be used for the good, can be used for the bad as well. That shouldn't however mean that that thing should not be produced/sold/used.
No, but I believe the state has a legitimate interest in regulating their sale and use.
But i'm not talking about guns in the abstract. I'm talking about the purposes that organizations involved in a black market gun trade would likely be putting them to. Collectors of firearms are not going to be using Silk Road 2. People who just want something to protect their home aren't likely going to be using it, either. The issue is the degree to which a black market, by virtue of it's purpose to evade law enforcement, would be used to facilitate criminal activity.
Or to extend your example further, no, we shouldn't ban cars, but we have every right to shut down chop shops.
Should we still not be frightened (or at least concerned) if what's being transacted is child slaves, or kidnapping/murder for hire, or guns for the mafia? There are a number of very unhealthy things which can fall under "voluntary transactions between individuals"
I'm not making an argument against anonymous marketplaces by any means, just against the idea that black markets are necessarily benign by definition, because they exist outside the reach of the state.