What this doesn't say, is how they tracked the guy after the intercept. My guess is that they somehow cracked more of the security than we have been lead to believe...
Even if the buyer had been strongarmed by the feds into cooperating for a deal, I don't think they'd have much useful information to catch the seller with, apart from an account that the seller trusted that they could use to order more packages, and they would have used his testimony to link the activities of Sadler/White to the NOD account.
edit: yeah, read the whole thing. it's actually pretty simple policework.