Multi Vendor Marketplace

I'm looking to build a digital goods marketplace something like [login to view URL], [login to view URL] or [login to view URL] The functionality will be really simple. Seller will be able to sign up, upload their products (and descriptions) and when buyers buy their products they will get a % of the sale in form of credits - when they accumulate enough credits, they will be able to withdraw the money. You can check out and see how [login to view URL] works and you'll see what we need. We already have the front end design ready in css/html format. I know there are a lot of similar scripts on the market. I'm looking for something that uses either the wordpress or joomla cms or a stand alone script that will run on php/mysql. I need just basic functionality and basic design on it - already have the front end design. My budget is $800 max. Please NO GENERIC BIDS - if you did something like this before please show it to me, if not, please propose a solution, tell me what CMS/Script are you going to use (or if you have something already made) and if you have or there is a demo for it. We need this done quickly.