osCommerce is one of free platforms you can take advantage of to bring your e-commerce store online. This open source e-commerce solution is quite powerful and can handle all kinds of stores. In order to make it work right out of the box with WordPress, you are going to need additional plugins though. WP OSCommerce Product Display is a simple solution that allows you to show products from your store on your WordPress blog.
The idea behind this plugin is simple. It displays products from your store. It has a widget that handles that. In order to get started, you should go to Settings > Product Generator and enter the database and appropriate URLs for your e-commerce store. Once that is done, you will be able to display products from osCommerce in the sidebar of your site.
I personally prefer running my e-commerce sites using WordPress or Magento. For osCommerce users, the above plugin could prove useful though.