This is, again, not how things work. A server operator can be charged for intentionally facilitating e.g. money laundering but you as a customer have 0 obligation whatsoever, besides ensuring that the service you're using is not going to simply run off with your money.

That's your legal theory, but I think the way things actually work in the US is quite different from how you think they should work according to libertarian principles, or even according to the laws as they are written. Being a fence, buying from a fence, even selling to a fence can land you in trouble.

Any sort of economic dealings with a person you should reasonably have known to be engaging in criminal activity, is a legal risk.

I don't think you find very many registered companies whose stated purpose is running a tumbler or otherwise "providing economic anonymity" - at least not in the west.

It's not libertarian principles, it's principles of US law, and in most places in the world for that matter. The reason being or buying from a fence is illegal is because it's explicitly criminalized. You can read the legalese here: https://www.law.cornell.edu/uscode/text/18/2315 Without that law it'd be 100% legal.