If you want to turn your Chamilo portal into an online shop for courses... you can't. Well, you can't do that directly, but there is an easy way to avoid that problem: using a real online shop system and connecting it to Chamilo.
Prestashop is a very popular, free software, online shop system that will allow you to setup your own online shop in a few hours.
Once you've got your shop installed and configure, go download the Chamilo plugin for Prestashop. On the plugin's wiki, you'll find a very extensive and simple installation guide to help you through the configuration. Once it's all configured, you'll be able to charge students to access your courses.
At this time, the plugin doesn't work with sessions, so you will sell an unlimited access to your courses, but we hope to have this plugin extended in this direction in the future.