check out wordpress (yes wordpress). It's coming of age as a CMS, and there is an awesome free e-commerce plugin at http://www.instinct.co.nz/
I'm currently using it for the first time to help a lady setup a small store selling soap. I think it is perfect for smaller e-stores, and for larger stores with > 1000 products I would recommend joomla with virtuemart(cost about $50 i think). Both are great php scripts (especially wordpress).