My 2 cents: The element of trust is primary for any financial transaction. There's a reason why eBay Amazon et al employ thousands for conflict resolution b/w buyers and sellers. I wouldn't mind going on SilkRoad for acquiring something which I otherwise cannot but for items which are available elsewhere, I don't mind giving up tiny bit of control while transacting on a commercial platform like eBay.
I think the relationship is desirable, which is why some people will pay a higher price to have a relationship with a vendor, with the understanding that if s transaction goes wrong, the vendor won't balk at fixing the problem. There might also be an expectation that one party or other will extend credit for a few days.
In contrast, there are touch-and-go relationships all the time, like at convenience stores. You can't have the exchange without being present for the entire transaction, because neither party trusts the other.
Escrow services would have to be an integral part of the architecture.
The buyer pays the vendor via an, initially, unsigned multisig 2-of-3 transaction, and the goods are sent when the buyer, the vendor and the escrow service each hold a copy. When the buyer receives the items, he signs the transaction and sends it to the vendor, who signs it and it's valid.
If a dispute occurs, the escrow service will have to decide the case, and side with whichever party it finds most credible. A reputation system for escrow services will be as important as one for vendors, as far as I can see, and they need to be compensated for their service, of course.
