1. A data processing system for making a trading market in at least one security in which the system proprietor is acting, as a principal; said system including means for receiving trade orders for said at least one security from system customers, said trade orders including fields identifying the stock to traded and characterization of the trade as customer purchase or sale, and the number of shares for the transaction; means for retrieving and for storing operative bid and asked prices for said at least one security; means for entering and for storing order qualification parameters, said parameters and said stored prices determining which received orders are qualified for execution; means for storing data characterizing position, cost and profit for said at least one security; qualifying means responsive to said received trade offers and said stored prices and order qualification parameters for qualifying a trade order for execution when the received trade order fields do not violate the stored prices and qualification parameters; means for executing each trade order qualified by said qualification means; and post-execution updating means for incrementing said stored position in said at least one security by the amount of a trade order upon execution of a customer sale and decrementing said stored position by the amount of a trade order upon execution of a customer purchase and means for updating at least one of said stored cost and profit upon execution of a trade.