Shopping Cart Info Required

I have a customer who need a shopping cart with about 6000 items with fully functional backend admin area that is fully featured using MS sql server for the database

In the future we will need to take the order and send it using xml to the customer supplier so the order can be processed directly, While charging Credit Card on our site.

We will need to be able to customize any portion of the shopping cart. (So need Source code)

Any Idea about good ready to go application/Product .

Regards,

GogiMicrosoft Commerce Server would fit what you need. Whether you can afford it or not is another issue.
http://www.microsoft.com/commerceserver/