  We explore the effects of social influence in a simple market model in which
a large number of agents face a binary choice: 'to buy/not to buy' a single
unit of a product at a price posted by a single seller (the monopoly case). We
consider the case of 'positive externalities': an agent is more willing to buy
if the other agents with whom he/she interacts make the same decision.
  We compare two special cases known in the economics literature as the
Thurstone and the McFadden approaches. We show that they correspond to modeling
the heterogenity in individual decision rules with, respectively, annealed and
quenched disorder. More precisely the first case leads to a standard Ising
model at finite temperature in a uniform external field, and the second case to
a random field Ising model (RFIM) at zero temperature.
  Considering the optimisation of profit by the seller within the McFadden/RFIM
model in the mean field limit, we exhibit a new first order phase transition:
if the social influence is strong enough, there is a regime where, if the mean
willingness to pay increases, or if the production costs decrease, the optimal
solution for the seller jumps from one with a high price and a small number of
buyers, to another one with a low price and a large number of buyers.
