Long story, but pretty much comes down to where and when was the first time anything about the silk road was ever posted online anywhere?
It was on the shroomery. Dred had just created the silk road, but what does someone do when they've created this marketplace and no one knows about it?
He went on the shroomery posing as a prospective buyer posting something like" hey guys, I just heard about this darkweb marketplace called the silk road. It lets you buy and sell drugs anonymously with bitcoin, ever heard of it"
When the feds were looking to bust him, one thing they did is go in reverse chronological order to see who first posted anything about the silk road. It was obviously by him, on the shroomery.
When he made that post he did so without using tor/a vpn, so they had his real ip.
The feds used his handle on shroomery "altoid" to see someone with an identical name also posted elsewhere looking to hire an IT specialists, although not illegal, it was linked to his gmail.
The first and only time he mentioned the silk road or drugs, when not anonymous was on the shroomery.
