I know you don't want to hear it, but Magento, while massively heavy, is going to be the best bet. I think it offers the cleanest way for front end sellers to manage their content.
The downside is, overcourse entirely too heavy, a pain to customize, and uses their own "wordpress" style function calls, but if you are creating a platform for a client that isn't exactly computer saavy then its probably your best solution.
Depending on time though, you could always create your own, then sell it 🙂
I know its not what you want to hear, but you kind of cut out all of the main e-commerce carts.
Sorry no help here.
