Recently what was called "operation ononymous" shut down many dark web sites. Most prominent victim was probably Silk Road 2.0, a darknet drug market platform. One might consider it the ebay of drugs (and other shady dealings).
Not unlike any other startup or entrepreneurial venture a high degree of risk would be involved in actually running such a platform.
Now, solely as a thought experiment I do consider 2 things in such an operation crucial.
1. Technical ability
2. Offshore financing
Technical ability is a biggy. Of course, any dork can buy a Wordpress template and pull up an easy shopping system and upload it onto a darknet webhosting service.
While the Tor network does provide some measure of anonymity by itself, it clearly is not sufficient just to use an .onion site and hope for the best. Some serious server mojo is needed, but that alone will probably not do the trick. Intimate knowledge of the Tor network and its limitations along with a high availability, some serious pen-testing skills and the endurance to closely monitor site activity probably would not hurt either.
According to my estimations this ist he first point where it starts to break down being a one man enterprise.
You might be asking, what offshore finance has to do with it. For any money to actually become useable in any way it will have to be washed and the benefactors will have to develop a way to get this income actually into their pockets. This will not only involve money laundring but also a way to store it and retrieve it without getting connected to the operation.
And after all, one would need funds to actually undertake this venture, i.e. server costs and the like and it would probably be a good idea to not get connected to these money flows either. Therefore offshore finance..
Too bad such a thought experiment cannot be transformed into real life without coming in conflict with the law, even if it was just out of curiousity or with the best of intentions.
