If the difficulty or fun of say blacksmithing nice items for the community. And you have it balanced out that you expect 20% of players be blacksmith. But then either 10 or 30% be blacksmiths either items skyrocket in price and make it unobtainable to most or the floor bottoms out making it not viable for most to engage in a market.
I think its solvable but itd require a lot of dynamic based systems to scale properly based upon number of players in each categories of life skill limits.
With that said, check out a game called Eco. They have life skill limits and force cooperation on small scale (30 day per server wipe). I think you can reasonably have 2 or 3 professions of 12 and people buy and sell from each other.