e-commerce order taking system

use cake, php, jquery, template whatever. I need a simple shopping cart order taking system. I know there are a ton out there, so if yuou already have one, then that will be ideal. I will need user login/registration different roles for customers (pricing level based on role) items with options order history account information paypal integration the customization needed is that it should hold "cash" or "credits" and orders get deducted from the credits in the system because there is so much open source and the fact that this has been done for others - budget is $20-50 if you can show me a working demo - you will win the job!