The beauty of Silk Road design is that the site being a FBI honeypot has been built into the threat model from the start. All identifying info sent between buyers/sellers is encrypted using PGP, Silk Road doesn't see anything.
That's not accurate. People are encouraged to use PGP for addresses and other sensitive communication there, but it is not enforced or directly supported by the system. There are still vendors who can't be bothered to figure PGP out (often relying upon dubious alternatives like privnote), and there will always be customers who don't care one way or the other.
Silk Road itself doesn't even have a discrete place for users to enter their public key; it is only by convention that most vendors put it at the end of their profile text.
