A dynamic market equilibrium management system is especially adapted for the sale of goods and services through an online buying group (referred to herein as a "co-op`) formed for the specific purpose of purchasing a particular product at (102) by defining a start time, end time, critical mass, any minimum number of units offered, any maximum number of units offered, starting price and product cost curve. As data is gathered from buyers, by means of their making binding purchase offers, the co-op is modified at (108) using the market equilibrium manager, so as to take into account market forces such as supply and demand for the item to be sold and their interrelationship with the purchase price for such item. When used with the online buying group, the dynamic market equilibrium management system permits dynamic, real time yield management decisions based on true market data. A graphical user interface receives user inputs for directly manipulating graphical display of data from a database on a display device and displays feedback dependent variable data on the display device, such as in the form of a changed numerical value in response to the user moving at least one data point in the graphical display.