PrestaShop is increasingly popular opensource ecommerce solution. It uses Smarty as a template engine so that lots of developers should be familiar with it.
Take a look in the web design portfolio for some nice PrestaShop templates.
These are some nice and complete tutorials.
http://www.daveegerton.com/prestashop-guides/Prestashop-Designers-Guide/Themes/modifying-the-default-prestashop-theme.html
http://www.techietips.net/creating-prestashop-13-theme.html
http://blogs.callosmart.com/2011/02/03/creating-a-prestashop-1-4-theme-from-scratch
