[dmk23]

The most effective attack is the easiest to predict.

It is human intelligence or infiltration by actors with a hidden loyalty. This has worked since the time immemorial for all sorts of security systems.

For FBI to bring it down they'll just need to repeat what they've done with Silk Road - pose as a straw buyers, get everyone they touch on record and use them to identify/arrest the users. Put out a request for a contract killing and arrest all responders. Bonus for putting a bounty on members turning in other members.

Even if this won't "shut down" the system itself, it would make it impossible to use safely.