I think a reputation system needs to get built or used.
I've been trading hardware over the net and we used Heatware.
I trust heatware and never had transactional problems in the 80+ transactions I did.
Of course, bitcoin might require something more sophisticated.
heatware is the greatest!
i started a buy-n-sell classifieds forum with a feedback system, it's
Bit-Coin-Trading.com. Having feedback points attributed to a user would obviously make that person more trustworthy. Both parties would have to be members of the forum, however.