Unless you have decent technical klnowledge of ow this stuff works, I SERIOUSLY recommend going with a paid provider. It does not have to be an EXPENSIVE provider, but the free options don't usually offer any technical support.
For example, OS Commerce is a pretty robust storefront. It is totally free. There are tons of free add-ons that extend it's functionality, free themes, etc. But, you have to know how to edit code to make it work. And if something goes wrong, you are on your own. Sure, there are forums where people may answer some technical questions for you, but getting answers takes time, and time is money when your store is offline.
If you really want to go the free route, I suggest Magento. Several people here have used it for smaller stores successfully. The free version does not come with support, but it is known to operate pretty well.
Miva is a decent store also. Very easy to use and maintain, but it costs $35 or more per month.
My hands down favorite is Volusion. They give you the ability to make a great looking store with lots of cool features. Very cutting edge. But that starts at $50 per month and there is a learning curve. They have lots of helpful video tutorials and great customer service, so the learning curve is not too bad. The one thing I don't like about them is their servers tend to slow down during peak hours.
Maybe others will chime in with better suggestions.
 | 
