Do you want a ecommerce centric site or a CMS site with ecommerce plug-ins? There is a distinction as the focus are different. Its generally harder to deploy and manage a lot of landing pages and squeeze pages on an ecommerce site than on a CMS.
There are dozens of ecommerce sites out there, I like OpenCart, osCommerce, magento, ZenCart and worked on others. Most also have commercially available templates / themes that you can tweak to give you a nice look and feel.
For CMS based ecommerce, depending on complexity, you can use WordPress, Joomla, Joomla, Drupal and DotNetNuke; all of them have plugins / modules that does an adequate job with ecommerce and you can find a bunch in their respective community that are free. If you don't mind spending a few bucks, I've build a lot of WooCommerce based ecommerce sites for small to medium sized clients in WordPress... Not a very steep learning curve and most can cover a few thousand SKUs easily.
The most complicated elelment is probably going to be your payment gateway. Most systems support PayPal, but there are numerous others including 2Checkout, AlertPay, Sage Pay, Payza, Liberty Reserve and more...
If you are looking for a roll our own, envato has a bunch of templates / scripts that you can pick up at a decent price (i.e. Code Canyon).
Hope this helps.