The present invention provides a method and system that allows sellers to communicate conditional offers to potential buyers. The conditions include prices that depend on the aggregate amount of goods or services that buyers collectively agree to purchase by a given time and date. The invention facilitates "demand aggregation", that is, aggregating demand by potential buyers (who may or may not know each other), for products offered by sellers. This invention allows sellers conveniently to offer "Demand-Based Pricing", that is, prices which go down as the volume of units sold in any given offer goes up. A seller can therefor offer volume discounts to buyers acting as a group, even when the buyers may not have any formal relationship with one another.