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.
JaJuMa-Market - All-In-One Multi-Vendor / Online Marketplace Solution. Various Business Models supported.
OpenCart - A free shopping cart system. OpenCart is an open source PHP-based online e-commerce solution.s
Sharetribe - Build your online marketplace business. You don't need a developer.