That's definitely a zen-cart site. Zen cart has lots of options but has a very messy structure to build upon. If you've got a great understanding of html, table based layout, and PHP then you shouldn't have much trouble making a theme for zen-cart.

OSCommerce I can't really comment on since I used it once and hated it. Didn't even finish with it and I found zen-cart. But, many people prefer OSCommerce to Zen-Cart

I also like wordpress and the wp-shopping cart plugin. It's pretty customizable, simple to use, and I'd say semi-open-source since there is a free and paid version. You're not supposed to modify the core of the free one I believe. It's made by a company called instinct.

There are some good ones for Joomla and Drupal too that I've heard of so you might check into building a cart in a CMS.