Abstract: A computer system provides a virtual marketplace in which an advertising opportunity (such as a television advertising opportunity) is expressed in terms of an audience model representing an audience associated with the advertising opportunity. The computer system provides potential buyers of a particular advertising opportunity with information about the audience model associated with that advertising opportunity. The computer system sells advertising opportunities to buyers using any of a variety of transaction semantics. A buyer who purchases a particular advertising opportunity purchases the opportunity to present an advertisement to an audience represented by the audience model associated with the purchased advertising opportunity.