Over the years, the FBI has taken down a number of high-profile black market and hacker-run websites, including the infamous SilkRoad that was hosted in the Dark Web. That site let you buy everything from drugs to a hitman, and did hundreds of billions of dollars in business.
Now the FBI can put another notch in its belt, and it's a big one. Along with law enforcement in 19 other countries, the FBI just shut down the Darkode.me forum and arrested 70 of its users. The reason this is such a big deal is because it shouldn't have been possible.
Any hacker-run website presents a challenge to shut down. You have to trace where the servers are located, which is a feat in itself, and work with law enforcement of that country to shut them down.
If you want to arrest criminals using the site, you have to find a way to match their real-world identities to their online user names. It doesn't help that they're using anonymizing services to make tracing their connection to the site nearly impossible.
That's just for a regular hacker-run site with ordinary users. Darkode was a forum exclusively for hackers, who you would assume know all the tricks and how not to get caught.