FoxyCart API might be of interest to you. Online service for the checkout portion - you can build the catalog any way you like. Integrates into most common CMSs. I suspect there are others out there too.
I've used eCommerce Templates and like it as well. X-Cart was many moons ago but not bad. Ran on the Smarty Template engine and you could run a dynamic site or have it publish the pages as static HTML.
The only downside to any of them is the learning curve before you discover it doesn't quite do what you want or will take far more than you expected to implement some feature. That's one reason I kind of like the online checkout services.