It would be useful for somebody to set up a place where bitcoin businesses can be rated, where people can give feedback and reviews. Of course, with places such as these, you have to watch out for people shilling for themselves, building up fake reputation to scam unwary customers. It is quite a dilemma.
This is the tricky part, though. If you go with a centralized site, like Amazon's Marketplace, you kill off one of the best features of online commerce: the ability for small sites to put competitive pressure on the big sites by offering lower prices due to their low overhead.
Otherwise, just relying on reputation and ratings requires much savvier consumers (and can still be manipulated by shills) and seems like it will heavily favor the bigger players. New sites will have a hard time getting off the ground since they'll be competing head to head with the scammers for customers who are willing to take a chance to get a good deal. Meanwhile, the bigger sites can jack up their prices and charge people for the "privilege" of not getting ripped off.