Looking for an open source decoupled eCommerce solution

I have a project where I need to build a niche eCommerce platform which will be licensed to merchants. I'm trying to find an open source framework or solution containing a merchant portal that I can take and customize to fit our needs. It needs to be decoupled from the front end, and exist on my server. Clients will login, configure their store settings, upload products, etc. A script will be generated for them to put on their existing sites.

I've searched and can't really find what I'm looking for. I'd really appreciate any tips or suggestions! I'm also open to a combination of solutions such as an open source CMS + some service to handle payment processing. Thanks again!