I think that initially prices will lower, with people who don't like selling things finding an easier way to get rid of all their stockpiled items. But eventually, with people being able to easily check what price other things are being sold for, prices will stabilize. That isn't to say that merchanting won't happen though. Merchanting relies on more than people who don't know the prices. People who want to sell quickly for fast cash, or buy something that they need quickly, will always provide merchanting. Or the occasional good deal for non-merchanters :D