I second Oscomerce. Our clients have had great success with it. The ecommerce forum seems to deal with oscommerce solely and we get people signing up just to ask for help on it. So much discussion on it that some searched on google end up at our forum - interesting.
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You can use the contribution "purchase without account" and you get the same effect. I would recommend taking that approach because it will allow you to rapidly deploy your site without spending alot of money and time.