Why Magento eCommerce

Magento eCommerce is open source ecommerce solutions developed in PHP and Mysql. It provides best performance compare to other paid ecommerce and open source ecommerce solution.

Many features puts Magento on top of all, like Multi Store Solution. You could literally create numbers of stores under one or multi domains. There are thousands other features attract you, no doubt. You could download feature list here.
http://www.magentocommerce.com/images/uploads/magento-feature-list.pdf

Magento Theme is very easy to design as per your requirement, and you could customize your theme at any level!
As Magento is Open Source, you must try once Magento. On LastTheme, you could download free Magento themes