WooCommerce - A freely available eCommerce plugin that enables shop facilities on your WordPress website. Functionality enabling extensions & beautiful themes available.
MarketplaceKit - MarketplaceKit is a platform for building different types of online marketplaces.
PrestaShop - Create your online store with PrestaShop's free shopping cart software. Build an ecommerce website for free and start selling online with hundreds of powerful features.
Sharetribe - Build your online marketplace business. You don't need a developer.
Shopify - Shopify is a powerful ecommerce platform that includes everything you need to create an online store and sell online. Try it free for 14 days.
Kreezalid - Marketplace building solution for small to midsize firms