If Joe wants to buy drugs, then he can walk into any bank and do a cash deposit to a BitInstant account, which gets dumped into MtGox and he can then move it around any way he wants, including online marketplaces. There's no identity that can be traced back to him except for the bank's video surveillance, and how long is that stored at a bank?
Without the surveillance, there's no way to tie any transaction Joe does back to his identity. The wallet, the address, the marketplace (if done right using tumbling and encrypting all messages) all leave no trace to a transaction.