For an organization to sell their products or services online there are several business models to be explored. Which business model (or a combination of several business models) is the best for you depends on your ambition level, market conditions, competitive landscape, etc. See 'What is an ecommerce strategy' for an approach of how to define your ecommerce strategy.
A number of business models are observed in today's ecommerce landscape. Frequently new business models (or a variation on existing ones) are emerging and some are also less succesfull (which seem to be different in different regions). Some examples of ecommerce business models are described below:
Outsourced online sales channel
In this business model the online sales channel is treated as a separate channel in comparison to other go-to-market channels. The operator of the online sales channel is treated as a wholesale partner with all benefits and constraints associated. The online channel is typically branded as your online channel and therefore perceived by customers as an online version of physical channels.
Sales of products or services via marketplace
In this business model you leverage the traffic generated on a marketplace to sell your products or services. In your business model you can define the role of the marketplace as focus to generate the sale or also include the transactional and fullfilment capabilities of the marketplace environment.
Inhouse managed online channel
In this business model you set up and maintain the capabilities required to support the ecommerce channel. You can decide to leverage third party services (eg hosting of ecommerce IT system by cloud provider), hire flexible people with specialist skills or scale advantages in other ways, but the end-to-end responsibility for the ecommerce experience is anchored within your organization.