I think it's scary that (a) these markets are here to stay, and (b) anyone with a computer and the most basic technology skills will be able to get any hard drug they can imagine delivered right to their doorstep.
Again, there's nothing to be done, really. That's just the way the world is now. But it's unfortunate.
Secondly, if Alice wants to sell crack, and Bob wants to buy it, I don't see why you or I or anybody else has the right to stand in the middle and say that's not allowed.
I accept that you think it's not good for Alice and Bob to be able to transact however they like, but you must accept that lots of people do not agree.
We're arguing that when drugs are ubiquitous -- as easy to access as pushing a button -- you'll spawn far more addicts than would have otherwise been created.
When an addict's life is destroyed by these horrific tools of pleasure we've created, are they going to take solace in the fact that they were able to use technology to enable their habit?
People become addicts for many reasons. One of the main ways is by being at a party among friends. No one expects to be addicted to this stuff, so they try a bit.
In the old days, they had one option: Get more from their friend, which may not work forever. In modern times, they can get as much as they want from the darknet markets. Bitcoin ATMs make it trivial, too. You don't even need to be vetted by Coinbase.
It's not just theoretical. People have witnessed the markets causing this.
I don't think the answer has ever been "make it so people can't buy the stuff", because that can't work. Instead, I believe that education, quality control, and help towards addicts would work much better.
The difference when dealing a human being is it's no longer a transaction between only Alice and Bob. There's now a third person involved who has every right to object to the transaction. Decentralised marketplace or not.
Am I proposing that the tools to solve human trafficking go away? I wouldn't put it that way. Lots of societal progress comes with trade-offs. There will certainly be challenges for law enforcement but I'm confident they'll solve them.
The world isn't going to descend into a chaotic state of everyone buying and selling everyone else just because free-trade becomes ubiquitous.
It's not everyone-else-in-the-world's responsibility to avoid building marketplaces where anyone can transact freely with anyone else.