Also some sites, like physical stores, offer "loss leaders". That's a product where the pric eis so incredibly good (sometimes even less than the store pays for the product) that customers end up looking around the store and buying lots more. Hopefully (from the store's POV) not checking to see if each and every item is a good deal - so the store can perhaps manage to "sting" the punters with some higher-profit-margin products while the punter is online/instore.