Good old-fashioned police work involving the postal system was how some Silk Road vendors were nabbed. It won't matter how secure your market is, if the authorities want you bad enough and you don't know how to do secure mail drops.
Also, adding levels of indirection only increases enforcement work linearly.
What you'd end up with are just a few organizations with the resources and expertise to outcompete other operators in terms of opsec and secure fulfillment, while wannabes and new operators serve as fodder for enforcement. Somewhat like today's situation.
Also, since enforcement depends on local enforcement, local corruption will increase.
There is no way they'll let digital black markets exist without creating a super-expensive heavy handed machine to attempt to stop it. Whether it is practical or not.
The problem is that at some point you must and want to interact with other people and each time you do that you increase the chances to leave a trace or do a mistake.