I find the market system to be an insanely good idea.
Take, for instance, the infamous Heart of Life. I admit that I purchase one as soon as I can afford it - they're just that good. Once I get my HoL, I never have to return to regen my health / mana ever again. Now, if a market system was implimented, the HoL's price would rise rather dramatically, causing less players to using, causing the price to drop slightly...
ahh.. it would be good times.