Merchants create ecommerce-enabled ad units advertising items offered by the merchants. The ad units contain functionality enabling customers to purchase the items by interacting with the ad units. The ad units have associated bid prices that the merchants agree to pay for sales through the ad units. The merchants provide the ad units to a broker. The broker publishes the ad units on web pages provided by publishers as comparison shop ad units that contain multiple ad units and functionality for navigating among them. A customer receiving a web page interacts with an ad unit to purchase the item. During the interactions, the broker dynamically updates the ad unit to conduct the transaction.