A seller server receives an offer which specifies a type of product (e.g. a seat on a flight), at least one condition (e.g. destination and date of departure), and an offer price specified by a buyer. The seller server also receives an associated payment identifier that specifies a financial account, such as a credit card account. The payment identifier essentially "guarantees" the offer in that the seller may receive payment from the financial account even if the buyer attempts to renege. The seller determines whether to accept the offer, based on various factors such as the offer price and the availability of appropriate products to satisfy the conditions of the offer. If the offer is accepted, payment is provided to the seller using the payment identifier.