This. Using a rented mailbox or something makes no real difference. If the police set up a sting (which is unlikely since nailing smalltime buyers is a waste of time when they can't possibly rat on the dealer) they will simply camp out at the mailbox and arrest you when you open it.
The PGP is there to ensure that the only person who sees your address is the dealer. If it wasn't for PGP then there could be someone intercepting the messages and fuarrrking over all the buyers on the site.
OP, your paranoia is both expected and healthy.
Look at it this way: It's safer than buying in person. In an F2F transaction it's very easy for law enforcement to prove that you were involved with the dealer and subsequently force you to act as an informant. With Tor deals they have a much harder time and they know that you don't know soykaf about the dealer, so arresting you for a misdemeanor charge would be a huge waste of time.
I wish to point out that the Silk Road bust (at the time the largest market on Tor) only resulted in the arrests of 18~ people. >>4373
Nobody is going to do that sort of thing in the US. Fortunately US mail is protected by the fourth amendment (if you live in a country without similar laws then I guess you're out of luck) . They don't have dogs sniffing your mail or x-ray machines scanning packages. The only time that sort of thing is brought into the equation would be if they have reasonable suspicion that the package contains something prohibited.
It's FAR easier to deny knowledge if a package shows up on your doorstep. Anyone could send you anything in the mail without your consent. In the event of a sting you can play dumb and pretend you dindu nuffin. After all, your PC (with the incriminating data) is encrypted, right?
If you get caught picking up a dead drop it's pretty difficult to get anyone to believe that you were feeling around under the park bench for some tasty (used) chewing gum and not a package of drugs.
I hear that people in Russia and some other countries use a dead drop system because their postal service isn't safe. I suppose it's workable, but not ideal.