5. The method of claim 2 , further comprising: obtaining, by the computer system, user profile information associated with the user, wherein the user profile information indicates a predetermined set of sellers specified by the user, wherein selecting the seller comprises selecting the seller from the predetermined set of user-specified sellers indicated by the user profile information.
2. The method of claim 1 , further comprising: selecting, by the computer system, a seller from which the product or service is to be purchased; and obtaining, by the computer system, seller information associated with the seller, wherein completing the purchase transaction comprises completing, without further user input after the receipt of the user input, the purchase transaction based on the seller information, the payment information, and the shipping information.