A linear credit demant curve
Let us now comment on the value obtained for ?. Note that the broker who gets the information (e.g. a buy order from a liquidity trader) knows that it does not come from an insider. If this order were fully transferred to the market, the price would rise, since unlike broker-dealers the other agents do not know that it comes from a liquidity trader and thus revise their price estimates upwards. The broker-dealer takes advantage of this discrepancy between the market price and his own best estimate, filling the client’s order in part out of his own portfolio. He is aware that the sale will lower the price, which is why he does not fill the whole order, for if he did so, this would perfectly offset the client’s demand and the final price would be unchanged.
Thus the broker-dealer behaves like a monopolist facing a linear demand curve, and offers half the quantity demanded, so that the price is driven below the marginal cost.