Sorry to be late to the party on this one, but a good option, although not free, is the Stiva Shopping Cart by phpjabbers.com. I’ve integrated the cart into sparkle in THIS SITE and it works very well. It’s also fully responsive. (go to the bottom of the page and click the “Take me to the Shop” button)
The cart itself is a PHP script that you install on your web server in a sub-directory. Once installed, it has it’s own admin interface for adding products and images. This interface also generates the code you have to add to your sparkle site.
The cart comes with payment gateways to PayPal and Authorize.net as standard. It also has options for collecting card details (over a secure connection) so that retailers with a terminal can process payments manually. Additionally, you can simply invoice and use bank transfers for payments.
If you’re not too handy at installing PHP scripts, the company will do it for you free of charge. However, your hosting must have PHP and mySQL facilities.
If you wanted to try before you buy, you can check out the demo on the developers website. If you go to the demo admin area, you will be able to grab the code from the demo and add it into your sparkle site. This will allow you to preview what the cart will look like and show you how simple it is to add it to Sparkle.