This is a lead. You can figure the percentage of responses that become leads, and the cost per lead. The clicker buys the product by giving a credit card number and authorizing the purchase. This is a sale. You can figure the lead conversion rate and the cost per sale.
Advertisers are only responsible for the first two steps.
They can get people to visit their site, creating impressions. They can encourage people to click on your banner, creating a response. The last two steps are up to the marketers on each web site. Viewers can and do get lost at each step along the way. As direct marketers we believe in targeting.