I have never understood the thinking behind this. Equated to street narcotic sales, this sort of debate is like expecting that it is solely the responsibility of your average drug dealer to protect the buyer from law enforcement. Both sides take their own steps to protect one another if sales are to continue. Obviously, this requires a certain level of trust. The main difference in this scenario of course is anonymity. The buyers of the Silk Road do not know the identity of their vendors, yet vice versa, the vendor has the name and address of the buyer. This is where that same level of trust has to come into play. The market provides a medium for the exchange to occur but so long as the correct precautions are taken on both sides, that medium should not know the identity of either party. PGP is the obvious and most elegant way to perform these transactions. If a buyer is not willing or is not knowledgeable enough to use it, they are their own worst enemy and would remain so under multi-signature transactions. The current system isn't broken so I see no need to change it.