WooCommerce is the best WordPress Plugin to build an online store.
WooCommerce is an e-commerce plugin from WooThemes and is free to use.
“Where is that snake in the grass?” I hear you thinking, but there isn't one. WooThemes makes money from the paid ones plugins en templates.
The plugin has now been downloaded more than 2 million times. More than 10% of all web shops/online stores use WooCommerce. This means that the plugin is now used more than Magento, Zen Cart and OpenCart.

When to WooCommerce and when not?

WooCommerce started out as a plugin for WordPress. Today, WooCommerce is part of the company behind WordPress. You will always run WooCommerce within an existing WordPress installation.
WooCommerce is usually used by small to large web shops. It is free to use, there are many tips & tricks available online and products are easy to create and customize. Adding more extensive functions can be done using (often) paid plugins. You can purchase these plugins through Milcraft.