There are certain websites that are only accessible through special proxies and through special techniques. These sites used to only be known to certain ill-willed groups. But like anything else, given time law enforcement and others find out about them.
There are online message boards and online storefronts in which cyber criminals buy and exchange information ranging from lists of stolen credentials to info on how to exploit certain types of devices.
The “good guys” when they find these sites don’t necessarily rush in and just shut down the site. If they did, the people involved would just move somewhere else. Instead they can monitor it and use it to build other cases against those involved. CC companies, and CRA monitoring services watch these sites for lists and compare it to what their members report. For example, myFICO offers this and asked for why email addresses and other sensitive items I want them to watch for including old passwords. If they find it, they tell me so I can be aware of it. I might choose to close the email account or change the password, etc.
