How can illegal transactions via the "dark web" take place without police awareness or involvement?

A few years ago local Ocean City Maryland police did make some arrests via Craigslist ads for escorts that would be “passing through” the Ocean City MD resort area and were booking dates in advance. Don’t know if they are still going after them these days. All things considered I have to slightly admire prostitutes with enough marketing savvy they are booking dates weeks or months in advance.