You can easily create an EC site by installing a module for EC to an open source CMS, "Drupal".
Drupal has been introduced by the world's well known brands and products for their EC sites. Drupal has a module with a strong E-commerce feature called "Drupal Commerce". You can built an EC site simply by installing the Commerce module to Drupal. The Commerce module provides all the features you generally need for EC site, such as shopping cart, product management, order management, and inventory management. You can use it only by adding it to Drupal.