BuySell shopping script customisation

We are looking for an individual who can help us customise an existing ecommerce script from the company Agriya called Buy/Sell. Our aim is to create a multi vendor ecommerce website. The code is clean and commented well, developed with PHP and uses the Smarty template engine. 90% of the website is done, we just need the following amendments done to get the site live: 1) Replace existing per item shipping module with a basic weight field for a table based shipping matrix. 2) Calculate shipping on checkout page based on the weight implemented above. 3) Customise the Add Item form (which is currently split into 5 sections into a 1 page form 4) Update the Checkout process. Currently the script forces you to checkout from each vendor individually but we would like the ability for one checkout button for all vendors. 5) Currently the only option for the payment gateway is through Paypal. We would like to include the option of direct credit card payment and also Manual/Cash on Delivery option. 6) We would like to include the ability to have variations on products (eg: clothing items would have Small, Medium, Large) within the same item without having to create individual products for each size. 7) Separate sign up pages for buyers and sellers. You can see a demo of this script on [login to view URL]