The reputation system should take that into account. You have to be careful with people who have a low reputation, but you could sell more expensive items to those with a high reputation and have a dispute center that would show how many times a user didn't receive the goods they wished to.

A buyer must give the money and then the seller send it. They should be required to have a tracking number on the item sent if it is above a certain dollar amount. The seller then becomes the one who needs to be trusted and not the buyer as they have already given the money.

Since it is visible on the blockchain, if a seller has 3 unresolved flags on their account, maybe their account should be frozen until they resolve any differences with the buyer.