Zen Cart is an open source e-commerce solution based on the PHP programming language and a MySQL database. The Zen Cart home page provides a great overview of the project. Not only does Zen Cart offer a very long list of features, the system is designed with both store owners and web developers in mind. There's no sacrifice of usability or power.
You get everything you need to maintain a professional, robust shopping cart web site for an unlimited number of products. Products, pricing, shipping, newsletters, sales and more are managed by the store owner through the administration area. The shopping cart is already set up to receive payments from any major credit card and several popular payment gateway services are included.
The great thing about Zen Cart is that anyone with basic computer skills can manage their own shopping cart. It is ready to go right out of the box. Installation is automated and the administration area is very user-friendly. All you need is a merchant account. One caveat: unless you want your store to look exactly like hundreds of others that use Zen Cart, invest in some professional web design services to customize the look and feel of your web site.
There is some basic self-help available in the Zen Cart support forum as well as satisfactory documentation on the web site. You may that customizing your shopping cart will require professional assistance. Other than personal preference, purchased solutions will have better customer support and more targeted solutions may be available for certain industries.