It would require too much oversight. Silk Road is already understaffed as it is, there just isn't the time or manpower to devote to this. It would cut down a tiny bit on the scamming though, if nothing else, but honestly guys: that's why vendors are supposed to put their real tormail address in their public keys. I don't know about anybody else, but my gpg keychain is gonna be my savior if SR goes down -- before everyone runs to the next one, anyway.