An optimized and safe trader-centered ordering system and a service method thereof are provided to prevent a seller from causing dishonor intentionally after the seller receives payment for buying goods from a plurality of buyers by enabling the buyer to inquire a financial state of the seller. A seller client(100) generates seller history by inputting a company name, an address, and records, such as a capital, the number of employees, capacity, a trusted transaction ratio, and real estate, capable of estimating a credit level to a history input window. A buyer client(200) generates purchase request data by inputting quantity, a unit price, the credit level of the buyer, a payment mode, and a desired delivery time to a purchase window, and selects one seller history for making a contract by searching the seller histories corresponding to the purchase request. A transaction server(300) receives the seller history from the seller, receives the purchase request from the buyer, and processes the transaction based on the seller history and an online contract window by providing a part of the seller histories corresponding to the purchase request to the buyer client.