"I find my greatest pleasure, and so my reward, in the work that precedes what the world calls success."
Expanding your store to add an eCommerce element involves a fair amount of work from both the developer and the client. One should look upon it in the same way as normal store inventory. Products and product options need to be added, pictures, the words that sell them, descriptions and stock movements.
Typically, a developer will set up the site and theme, get your online store in functional order, and add enough products to display the online version of your store structure. This is the point at which training comes into play. Although the developer should always be available for support and guidance, the main inventory, how it's added and kept current should be the clients responsibility.
It is important to undestand that the creation of a website doesn't mean handling all of the store's products. However, there is always the possibility of paying the developer (or someone else) to input the products. This is a good alternative solution if busy schedules do not allow the client to add the products themselves.
While the customers will see one side of the site, all of the clients work will take place in the administrative section. This is where all the products and associative tasks are done. As an add-in, there is also a possibility to merge the online store with inventory and finance handling of your normal store. This, in more cases than not, would take some customization, but it could be done.
It is always nice (and preferable) to handle all of the stores administrative functions in one place.