Abstract: An electronic open market system for preventing contents from being stale, for eliminating differences in an amount of available information, and for eliminating obstacles to computer system linkages, by means of a method, comprising the steps of: (a) storing public data enabled to be viewed by standardizing attribute information about the same products (or services) supplied by a plurality of suppliers so as to compare the information with each other and non-public data including transaction rules set for each combination of a supplier and a buyer as a database for an agent; (b) the buyer's reading and viewing the public data from the database via a data communication network to select a desired product and to apply to the agent for a transaction of the selected product; (c) the agent's reading the transaction rules depending upon the buyer having applied for the transaction from the non-public data, making estimates, and presenting them to the buyer; (d) the buyer's determining a supplier and purchase condi