Well, have you looked at Magento?
Link:
http://www.magentocommerce.com/
Its really good, feature rich and just generally better than most open source software available. However, you will need
good hosting,
a solid grasp of php/html and
a load of patience to get used to the template system.
Learning to code magento is a very useful skill - lots of agencies (here at least) are going with this option.