Silk Road relied on Bitcoin and escrow among other things, and presumably the successor will do as well or better with payment security. So it's not quite as simple as "go where the money flows and arrest anyone selling". How do you propose to trace payments? Maybe some careless operators will be nabbed but they are already harder targets than the guy on the street corner, and those following best practices will be more elusive than that.

Kudos to these guys for undermining victimless-crime laws.