If you want to buy or sell something illegal, you should do it on the Dark Web. Note: There is of course no public shop address, the Dark Web cannot be found via Google or Ecosia. Special browsers are required, so-called "Tor" browsers (Tor is a network for anonymizing connection data). Transactions between parties are made between anonymous parties, identities are not revealed. Bitcoin or other crypto-currencies are used for payment, so that payment flows cannot be traced.
But the dark web is also relevant for whistleblowers or regime critics; because it offers a high degree of security and anonymity for the exchange of information. For example, major newspapers such as the New York Times have set up pages in the Dark Web, so that informants can provide highly confidential data and information at low risk. In fact (and somewhat surprisingly - but only at first glance), "Tor" was developed by US authorities to provide agents with the means to securely transmit data/information.
A very good video with an overview of the history and dimension of the Dark Way (duration: 11 minutes) can be found here: Exploring the Dark Web