1. A promotion system supporting a plurality of buyer computers and a plurality of seller computers, the plurality of buyer computers used by a corresponding plurality of potential buyers, the plurality of seller computers used by a corresponding plurality of sellers, the promotion system comprising: a promotion management service that stores information regarding each of the plurality of potential buyers; the promotion management service receives from a first seller of the plurality of sellers via the first seller's computer, promotion data related to an item being offered for sale by the first seller; the promotion management service presents promotion data via at least a subset of the plurality of buyer computers; a promotion email service that, based on the stored information, sends to a subset of the plurality of potential buyers an email including the promotion data related to the item being offered for sale; and the promotion management service, responsive to input received via a first buyer computer of the plurality of buyer computers, communicates to the first buyer computer promotion information for storage on the first buyer computer, the promotion information to be used to support completion of a transaction at a physical seller location of the first seller at a later time without receiving by the first seller at the later time of the promotion information from the promotion management service.