There are a whole bunch of free open source ecommece setups out there but few that are really simple as they've all grown over the years. Thats not to say you can strip back the feature set for you clients. A great opensource PHP based one is OSCommercce (www.oscommerce.com). Real easy to install and use and there are a whole bunch of templates to choose from if you don't even want to design something.
Other than that a much more fully featured one is magento (www.magentocommerce.com) but it may be a little overkill for what you are trying to do!
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