The markets themselves wern't the weak links. I think most of the people that get caught were caught from making silly errors like Ross was. It's not that hard to track the sellers down when you have to go and post the items. You can find fingerprints on packages and narrow down where they're sent from. Then you just stake out the places and wait. Eventually you'll get a catch.
Yes, good old fashion police work will still catch "criminals" , but what happens when you have a market of thousands of small time dealers competing in a thriving market with little assets each to seize? They can make a few examples at a loss but will eventually run out of money and have to focus their attention on the larger criminals.
Police can never take down all the crime even in the real world. The majority of crimes or drug dealers go unpunished because they just dont have the resources or intel to cope with it. They usually just work with what info they have and take down what they can when someone slips up. It's hard work but they won't stop doing it.
Some small time dealers who make mistakes will indeed be caught as well, many small dealers that follow a few precautions will be able to thrive and good for them. I will only be selling legal items on openbazzar as I would have to be a complete idiot to be so outspoken and try and sell illegal items.
Depends how good at keeping yourself anonymous is, but I wouldn't like the stress and paranoia or attempting to do so that's for sure.