This is far from an exhaustive or comprehensive list, but this page illustrates some of the most common ecommerce business models and how they work.

Basic idea: the seller is listing inventory for sale they don't actually own. They buy the inventory once a customer pays, and then ship it to the customer.

Basic idea: Seller buys inventory at a discount from a retailer or online store, and then sells it at a higher price somewhere else.

Basic idea: Seller negotiates a deal with a wholesaler, distributor, or manufactuer to purchase large quanitites of products in bulk at a discount. The seller then sells them online.

Basic idea: Seller invents their own product or innovates on someone else's. The seller then has it produced, markets it, and sells it online.