Cost vs Benefits
The costs of open trading is the possibility of inflated pricing and that the markets would be controlled by large players.
The benefits of open trading is that wider fluctuations in pricing create opportunities for traders to actually profit outside the current range and the ability of players to transfer larger resources than 16:1.
The costs of open trading are the objections to it and are generally raised to protect the prices of items becoming outside the reach of too many players, particularly the smaller ones. But I think those raising this possible scenario miss two important factors. First, trades are localize and Fellowship wide. In other words, the 150 or so with which I can trade (those within my range of discovery) and my FS mean that there would be only a small fraction of players I could effect if I were to try to control pricing. Even at the very top tier the number of players is probably around 500, a very small fraction of the number of players on the server. In other words the market leverage and scope I could enact would be insufficient to actually the markets outside my local influence. And while it is true that large players could collude the number of those large players needed to effect the general market and the scope of their coverage (the number of players they could influence by a concerted effort) would have to be quite high. Simply put, while not impossible, it would be highly, highly unlikely. Second, "inflated" prices would not have a long term sustainability. Anyone a few squares away and outside the direct influence of the "evil consortium" (lol) would be able to purchase from other players outside the scope of the large players influence and while it may be likely that those players may raise their prices to some degree, given the flow of the game, it would be a short term effect.
On the plus side though there are some nice benefits. First, if I want to transfer 100 T3 to a player he/she would need pay up 1600 T1 or 400 T2 items, for instance). The ratio's are artificially set. If the market was open the player desiring the resources could simply put up a 1 to whatever trade and I, as their benefactor, could pick it up. This would be a great way to assist really small and somewhat small players and thus encourage them to remain in the game. Second, an open market, while not perfect, would allow the players to set whatever price they wished for the trades, and thus, during shortages, those having the presence of mind to foreseen the shortages coming and to save up the resources, could actually do well. This adds an aspect to the game which would make it more appealing to some players (myself included).
In the end the costs of a open market are constricted by the game mechanics and the difficulty of organizing and maintaining any sort market control over a wide scope of players, while the two benefits add interest to the game and encourage greater participation. More participation is always better, in my opinion.
AJ