What are the requirements for E-Commerce?

Technically, there are 4 major requirements:

1) An Internet Merchant Account
2) An online Payment Gateway (authorize.net, PayPal PayFlow Pro)
3) An E-Commerce enabled website: including Shopping Cart & Checkout
4) A Secure Connection to your Payment Gateway (SSL Certificate might be required)

Please see the Credit Card Processing Diagram by authorize.net for more information.