Like, I know the site exists, but how many people here know people who have successfully conducted commerce on it?
How are they keeping themselves from being exposed?
Then, they have to setup a trap in which the recipient knowingly takes delivery (and there can be no other way than the recipient signing for it, or else it would be entrapment or just mis-shipment, which is just as common) and catch them. All for a person who bought a small packet of drugs.
I think the Feds even consider such a thing a lost cause.
Unless they only buy virtual goods, there's a trail to follow.
There's nothing inherently linking the two transactions of "X bitcoins transferred from wallet #1 to wallet #2" and "Y USD transferred from Bob to Joe" - with sufficient care and redirection, you can make it very difficult to track down exactly where the bitcoins are being traded for real money.
It's the main driving force behind the price of bitcoins on the public exchanges, which makes for a rather novel opportunity for a market-moving attack I'll leave as an exercise for the reader.