A network transaction method is for consumers and users to trade a product via the internet. The network transaction method provides a must-buy or first-to-buy transaction mode and comprises the steps of exhibiting product information to allow one of the consumers to select the product; generating a transaction page and a sign-in page according to the selected product to trade, and the sign-in page being provided for the users to sign in directly or push a post to recommend other users to sign in; using the sign-in action to change a transaction price of the product, such that the consumer can end the sign-in action according to the current transaction price or product information provided by the system automatically, and finally allow the consumer to close the transaction and make a payment for the product to be traded. In addition, a network transaction system is also introduced.