The customer agent 9 transmits a search request 15a to a manager 4, which in turn transmits a search request 15b to a shop agent 5. The shop agent 5 transmits a product information service message 16a to the manager 4. The manager 4 transmits a product information service message 16b to the customer agent 9 and a product information service message 16c to a shop agent 17. Based on the thus monitored product information service message 16c, the shop agent 17 generates an associated information service message 18a and transmits it to the manager 4. The manager 4 then transmits an associated information service message 18b to the customer agent 9.