Beside Shop-Script FREE, there are two excellent opensource ecommerce software with PHP-based. Try them at OpenSourceCMS. There are ZenCart and osCommerce.
It's easy to install and utilize.

Simply free e-commerce software from Shop-Script FREE User Guide (PDF File - 1022Kb).

Visit the demo at here.
It's enough for first try on build ecommerce site.

A blog dedicated to slash-review webstore.