I'm currently in the process of building a multivendor ecommerce site built in PHP following the MVC architecture pattern. I have a fair chunk of the work done so far(I'm a designer/developer but this is starting to exceed my abilities), but I need some help getting the actual commerce part of the application together (shopping cart, order reporting, paypal adaptive payment integration). I have an existing codebase that I would like to use. I do not want to switch to Magento, or Drupal, or Joomla, or Wordpress, etc. If this is a deal breaker please move on.
To go into detail:
Seller accounts are set up where a Seller can add inventory from a predetermined list using the backend I've created. When Customers search the site for products, each product should list below it the Sellers that have the product in stock, with quantities, and an add to cart button. Then they should be able to add them to their cart, and be able to check out using Paypal. This should be done using Adaptive Payments as I'm looking to take a commission from each sale. Once the purchase is completed, the items should be removed from inventory, and each Seller should receive an order list with the Customer's information and purchased items. The Customer should also receive confirmation of the order, including the items purchased.
I'd prefer to keep code as clean and effective as possible. I'm not looking to make things harder in the future as I expect this application to need to scale.