1. A computer-implemented method for determining an electronic advertisement to be displayed, the method comprising: under control of one or more computer systems configured with executable instructions, defining an item grouping for a plurality of related items; associating the item grouping with a target audience that comprises one or more users likely to purchase an item from the plurality of related items in the item grouping; receiving bids to display to the target audience an electronic advertisement associated with an item from the item grouping; evaluating the bids received according to criteria including at least a bid price and a quantity of bids previously received from a given advertiser; selecting a winning bid from amongst the evaluated bids; determining whether a pre-existing advertisement exists for the item associated with the winning bid and, if no pre-existing advertisement exists, generating an advertisement for the item using components stored in an advertisement components store; designating the pre-existing advertisement for provision to a content provider when the pre-existing advertisement exists and designating the generated advertisement when the generated advertisement is generated; from a content provider, receiving a request for an electronic advertisement for display to a user, the request identifying the user associated with the request; determining whether the request is related to the item grouping based on a purchase history of the user; and providing, to the content provider, the designated advertisement for display when the request is related to the item grouping.