The three most important elements of an ecommerce platform are the features and functionality that let users:
Search the store and browse products.
Choose and manage the items they want to purchase by adding them to a shopping cart.
Check out and buy their products, including secure payment gateways.
Together, these are the three core functions that power a basic ecommerce platform. Many larger platforms in the market today offer many more elements to ensure a modern, personalized shopping experience.