Hopefully users will be able to sell too. Success of ecommerce site mostly comes down to choice ( number of products on the site). Allowing every one to set up a shop will increase your chances of success a lot. I would love to see an integration of paywithsteem with steem accounts so that steemians can log in to your site with their steem account and start selling products. Steem also have an escrow feature that you might be interested in. All the tools are there to build a badass marketplace :-)