For example, for a while most prostitution and sex work seemed to be online, on places like Craigslist right next to ads for used furniture and jobs. And it seemed to be really effective in getting prostitutes off the streets.
Now that those markets were shut down, I'm seeing here in Seattle we're having pimp shootouts on Aurora and the prostitutes are more brazen than ever. Going after Craigslist has had a negative effect on our cities and has increased crime, and I suspect going after SilkRoad has had a similar impact.
My point is rather that an online marketplace in the absence of decriminalization and reform can only provide a marginal increase in safety. Sex workers marketing on Backpage, Craigslist, Onlyfans, and IG still face a great deal of risk of violence, pressure from pimps, and prosecution by law enforcement. It's a deeply complex systematic issue which can't be fixed by a website.
For drugs in particular, darknet marketplaces primarily rely on unspeakably violent criminal enterprises upstream. The consumers, sellers, and communities implicated in this supply chain are all losers in this system. The cartels are the winners and the global "war on drugs" establishment are a close second place.