In my view the best way to do this and avoid most of the problems cited above would be to create a self-stabalising price system whereby every commodity has a set price that it naturally shifts back towards over time if nobody is buying / selling it. This would represent the npc traders balancing out the effects of the players so that no price ever fell too low or rose too high for prolongued periods. As for how large these balancing effectes would be, that could easily be adjusted.
Generally an interesting idea, but needs careful thought and some pretty complex maths to calculate how to set all the parameters to prevent crazy outcomes. I'll leave that to someone more qualified than myself...
Sir Stanley Nelson <span style="color:#000066">Charles Canning </span><span style="color:#000066"> Foreign Secretary</span>