After The Sale One firm purchased the names and addresses of people who had already bought several thousand dollars worth of services. The firm found these folks were more than ready to purchase even more services. The list was far more valuable than a list of interested prospects who hadn't bought yet. This drives home the old saying that 80 percent of your sales will come from 20 percent of your customers. People who have just bought are the customers who are most likely to buy again.
Some sites have a second order page come up after a customer has filled out the first order page.
These sites often get a second sale from someone who has just bought. Other businesses are gathering email lists of customers who have purchased various items. People who bought item A get a message about a good deal on a related item. Folks who bought service B get a message promoting an additional related service. Get Kevin's Marketing Plan Packages that pack a promotional wallop.