The Opinion WordPress theme fully supports WooCommerce WordPress plugin with which you can easily create your own shop.
Once you install and activate WooCommerce plugin you will get some basic layout and sidebar options to manage in Theme Options -> WooCommerce.
For detailed documentation and explanation how to use WooCommerce, we warmly suggest you to visit and learn from their official documentation: http://docs.woothemes.com/documentation/plugins/woocommerce