Does anyone know where I could begin to look to learn how to create a basic ecommerce payment system in C#.NET? I'd prefer to create a payment system where people enter credit cards and not depend on prepaid services like paypal.
Also does anyone know a gateway provider that has a marketplace type of experience, where users can transact between each other?
For credit card payment, you need to get a payment gateway provider. You need to create an account with them and become their merchant. The payment gateway provider will provide you a merchant ID to identify you, together with SDK and sample code to integrate your website with the service.
Basically these are things you need to do:
Post the payment amount, payment ID (generated by your system) to the https payment URL provided by the service provider.
User enters their credit information on the page.
Payment gateway redirects back to the landing page that you "told" them beforehand.
Retrieve the posted data from payment gateway to check the payment status, error messages etc.
This is not the end yet. You need to do settlement on daily basis to retrieve the list transactions that has been submitted to credit card company. It should be provided by the payment gateway provider as well. You will then mark the transactions in your database as "Paid" based on the settlement.
This is the rough idea of online credit card payment, contact them to find out more.