WooCommerce is the most popular WordPress eCommerce plugin. And it's available for free. Packed full of features, perfectly integrated into your self-hosted WordPress website.
We have implemented this and its a great way for a small to medium business up and running with an eCommerce solution. Woocommerce can be integrated into your existing WordPress theme or you can purchase a dedicated Woocommerce WordPress theme for approximately £30-70. Themes are available that enable you to produce a standard website with a dedicated eCommerce solution built-in. By adding featured posts and pages plug-ins your website will look great and provide a great platform to market your products and services.
Woocommerce has a range of extensions that provide additional functionality such as product data import and creating your own merchants using Product Vendor.