  We study the collective behavior of interacting agents in a simple model of
market economics originally introduced by N{\o}rrelykke and Bak. A general
theoretical framework for interacting traders on an arbitrary network is
presented, with the interaction consisting of buying (namely, consumption) and
selling (namely, production) of commodities. Extremal dynamics is introduced by
having the agent with least profit in the market readjust prices, causing the
market to self--organize. We study this model market on regular lattices in
two--dimension as well as on random complex networks; in the critical state
fluctuations in an activity signal exhibit properties that are characteristic
of avalanches observed in models of self-organized criticality, and these can
be described by power--law distributions.
