Probably simple application of old-style investigation techniques: "Follow the money". They were receiving payment via traditional mechanisms to which the DEA has access if needed.
Some commenters on this story are suggesting that a Tor flaw may have led to the bust, but that's conjecture. The DEA didn't hand over details of how they got the goods on these guys. As Sophos's Chester W said, could have been good old gumshoe work. Could have been a busted drug supplier who ratted them out. Can't say. Don't automatically blame Tor.
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