Disclosed is an online product sales system and method convertible between sales and auction modes. The present invention comprises: a seller terminal for receiving, as an input, the highest auction price, the lowest auction price, and an auction duration for a product, and transmitting the same; a product sales server convertible between sales and auction modes, comprising a product registration module for receiving, from the seller terminal, the highest auction price, the lowest auction price, and the auction duration so as to register and post the same thereon, a buy-it-now price reception module for receiving a buy-it-now price corresponding to the registered and posted highest auction price, a proposed bid price reception module for receiving a proposed bid price which is higher than the registered and posted lowest auction price and is lower than the registered and posted highest auction price, an auction execution module for awarding the bid to a purchaser who has proposed the highest bid price among proposed bid prices which are higher than the registered and posted lowest auction price and are lower than the registered and posted highest auction price, and a product sales module for, when the buy-it-now price reception module receives the buy-it-now price for a product, immediately selling the product to a corresponding purchaser at the received buy-it-now price and for selling the product at the relevant proposed bid price to the purchaser to whom the auction execution module has awarded the bid; and a purchaser terminal for receiving, as an input, a buy-it-now price for a product registered and posted on the product registration module and transmitting the same to the buy-it-now price reception module, or for receiving, as an input, a proposed bid price for a product registered and posted on the product registration module and transmitting the same to the proposed bid price reception module.
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