I myself ventured onto TOR in the past and it was like something out of a William Gibson cyperpunk novel, where you jack in and any sort of illegal item you could want is for sale - cocaine or heroin or guns or your boss killed or pictures of underage children or whatever else.
In amongst discussions of politics or other matters, TOR contains a bizarre online bazaar, a virtual souk of the illicit, illegal and immoral. All you needed was to install the TOR connection software and have ready access to the online-only Bitcoin currency. Now, whether such sites were genuine or scams or even some odd dark-humour I did not attempt to find out. All I can say is web sites existed which purported to sell such things and which provided details how to pay via BitCoin in advance.
Unlike eBay where shops and shoppers alike pride themselves on positive feedback, TOR's anonymity cannot be understated. After all, that was it's reason for existence. Who was selling? Who was buying? Nobody knew. The FBI wanted to know, however.
In the visible Internet, the one we all know, if you wish to publish a web site you could use your own infrastructure or sign up with a web host. It's the same on the dark web, and due to the complexity of self-hosting, many TOR sites chose to host with a provider, specifically The Freedom Host. It is this host who the FBI targeted.
The FBI state they are cracking down on pedophiles and have now compromised half the TOR network of sites including TORmail.
The founder of Freedom Hosting who has been arrested in Ireland and at the time of writing is awaiting extradition to the USA. This individual has been named by the Ireland Independent newspaper as Eric Eoin Marques, 28, with dual Irish and US citizenship. The Independent states an FBI special agent called Mr Marques "the largest facilitator of child porn on the planet."