If you just have a few products and don't need the full features of a shopping cart CMS then have a look into PayPal 'buy now' buttons. This is the easiest platform to get started with - all you do is copy and paste some code into your website (under the product description) and everything is taken care of for you. So you could get a Wordpress/Joomla/Modx site built and just paste the code in anywhere... very simple and all the book keeping is done through PayPal.
For full features go with Magento - hands down the best designs and is a relatively new CMS that doesn't have outdated code. There is lots of free templates but it does take some knowledge to customize the CMS so go with a pro if you want any extras.
If you have a social community site then go with Joomla because they are he best. It doesn't come with a shopping cart but the virtuemart add on is easy to use and give you everything you could need.
Hope this helps!