For example: sellers try to attract wary buyers by accepting a flexible X% payment in good faith, and X% in escrow, unlocked on delivery.
For expensive items, the market operator (or Bitcoin??) could provide a two-way escrow system, whereby both the buyer and seller set aside some funds, so that both sides are motivated to be honest. Imagine going on-line, paying a fully refundable deposit in Bitcoin on a piece of jewellery/car, meeting the seller in public, taking the item back with you, and then paying the remainder on-line!
Many Bitcoin Escrow companies exist, and I even talked about it at the end of the panel