INTRODUCTION One of the most time-worn statement of economic science is that “prices are such that supply matches demand”. In order to explain how this really comes about, one usually invokes a Walras auctioneer, who attempts to measure the supply and demand curves S(p) and D(p), that give the total amount of supply/demand for a given good (or asset), would the price be set to p. The equilibrium price p* is then such that D(p*)=S(p*), which maximizes the amount of good exchanged among agents, given the set of preferences corresponding to the current supply and demand curves . In reality, the full knowledge of S(p) and D(p) is problematic, and Walras envisioned his famous _tâtonnement_ process as a mean to observe the supply/demand curves. However, there is a whole aspect of the dynamics of markets that is totally absent in Walras’ framework. While it describes how a pre-existing supply and demand would result in a clearing price, it does not tell us anything about what happens _after_ the transaction has taken place. In this sense, the Walrasian price is of very limited scope, since the theory ceases to apply as soon as the price is discovered.