So every transaction can be traced to its origin. If I connect to the network through TOR and generate 50 BTC, and then buy some substance with it, it's very hard to trace that to me. But even if I bought coins at an exchange, and they, as required, have verified my identity, I can still do the following:
- create another wallet (possibly online, like instawallet)
- transfer some BTC to that wallet
- buy substance with that second wallet
- erase that second wallet
now I can always say in court that I paid some other guy for his service (drug withdrawal consultancy?) and that's him who later bought the substance. While obvious lie, can that be considered a proof beyond reasonable doubt that I indeed bought the substance?
The person you bought from could very likely have been FBI/DEA, and as soon as they've created enough of a network of people like yourselves who have bought 'substances' that they think they will be bust another Silk Road supplier, they will kick in your door and put you in jail.
If you think FBI/DEA aren't at least one of the sellers on Silk Road, you're an idiot.
You will get busted