If you're a start-up and looking for a MVP especially in an eCommerce vertical market, we'd love to be involved.
We'd stress eCommerce as `partnership` because building and running a high-end on-line store is a journey and not a quick out of the box product. Your actual shop or its foundation won't take long to set-up, but if all you want is a vanilla store then just choose 'Shopify'.
You start with a generic product suite [+ all the usual stuff] and then overlay on this, which is all done with custom PHP/HTML/CSS/JS. There will be a powerful backend CMS and this is great for maintaining products, but we're talking about the workflow of the site and one which is unique to you. This is where a professional Web Developer is required and not just a Web CMS Manager. You need both, of course, and the latter is probably you. We can also customise the back-end in extreme cases, but that's unlikely unless you are a huge eRetailer.
In light of some technical developments [improvements and the contrary] we are only taking on new projects with our custom vShop and this has a full backend PHP and API suite. The frontend can be written as you see fit, but for greenfield projects we use React JS. You can visit these on bitbucket as they are Open Source.
Our relationship with Magento has alas come to an end as their systems are just proving to be unworkable on a cost-management basis for clients.