Read the complaint PDF. Basically when they started finding stuff in the mail, they noticed most packages looked pretty similar. Went down to the post office where some of them were being shipped from, told them to keep an eye out for similar packages. Didn't take them long to track down a couple of people who were posting them, etc, etc.

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.