osCommerce Online Merchant is an Open Source online shop e-commerce solution that is available for free with a feature rich set of out-of-the-box online shopping cart functionality that allows store owners to setup, run, and maintain online stores with minimum effort and with no costs, fees, or limitations involved. It has a large community with many contributions.
The webstore system for ACE Retail POS is developed from osCommerce. Many features and options have been added, including a customizable template, configurable rotating banner, enhanced matrix/feature product support (size/color/etc), custom price and private website support and many other features.
Some sample sites here:
You can find from here how to turn an osCommerce site into a private site:
Private site contribution for osCommerce (2.3.1)