X-Cart – ecommerce shopping cart software:

X-Cart shopping cart software is based on PHP Smarty templates, this makes it very flexible and easy to modify. The data is stored in MySQL database.
W3C-compliant storefront design with a CSS-based layout.
You receive a complete source code and SQL tables for MySQL database to make them extended or customized easily.
X-Cart is a turnkey package. Installation is handled by web-based step-by-step wizard to make the system running immediately after receiving it.
X-Cart is search engine friendly. A merchant can specify meta tags and define custom URLs for product and category pages that results in higher search engine rankings.
Our experts are always ready to answer your technical questions about the software and handle installation & configuration tasks for you.
We also offer custom programming services. Every client can get a storefront with a unique look. We can customize the software to completely fit the structure of your business.
Additional software licenses come at discounted rates.

My recommended links:

http://www.martstreet.com
Their Help Center is like a perfect book.

http://www.threehosts.com/business/1st

http://www.threehosts.com/business/3rd

Hope this helps.