Free eCommerce platforms
1. OPENCART
My favorite platform. One of the best free open source e-commerce platform on the market. Fast, user friendly and plenty of usefull features, like coupons, marketing and much more! Twice faster than an WooCommerce Plugin of WordPress. Excellent for small to medium size business. Another great feature that OpenCart platform uses Bootstrap Front-End Framework.
2. WORDPRESS
Most popular content management system in the world. WordPress is an universal CMS and widely used as blog or ecommerce platform.WooCommerce plugin must be installed for using WordPress as e-commerce platform. Not recommended for large business. SEO Yoast - most important plugin on the market , which helps to create Google friendly content.
3. PRESTASHOP
Great free open-source platform with plenty of functionality. An excellent choice for medium size business. Note: Prestashop is free, but some modules can be expensive. If you don't know CSS, HTML, PHP i would rather go with OpenCart or WooCommerce instead of Prestashop.
4. MAGENTO
Big player for medium to large size businesses. Widely used to serve few shops in a single dashboard. With Magento platform you can do a lot. Marketing, SEO, advanced product options and much more. Unsuitble for small business.
5. X-CART
An impressive platform for small to large size businesses. There is a free version of X-Cart, which has all of the essential features for getting started with an online store, as well as such advanced features as newsletters, social-commerce sharing, and a sitebuilder for tweaking themes. X-Cart will provide free core upgrades for the free version. It also includes free extensions such as ShipStation to generate shipping labels, Magic Slideshow to create a slideshow on the site, and Diib Analytics—which provides predictive analytics for your store.