Check out the following article about Integrating PayPal Payments into ASP.NET E-Commerce Applications. this a very descriptive, useful article.

http://www.west-wind.com/presentations/PayPalIntegration/PayPalIntegration.asp

The process of integrating a PayPal payment involves several pages and HTTP POST backs.

one issue we usually face while developing an ECommerce application is testing credit card  payment system. so here is list of credit card numbers we can use to test payment system.

American Express: 3111-1111-1111-1117

Visa: 4111-1111-1111-1111 or 4222-2222-2222-2222

MasterCard: 5111-1111-1111-1118 

Discover: 6111-1111-1111-1116

remember that  CCV number and Name does not matter but you should keep the amount bellow $500 dollars and also make sure expiration date is not today but in the future.

Example:

Visa:4222-2222-2222-2222

CCV:123

Name: Vahid Taslimi

Expiration Date: 0110

Amount: $486.65

Hope this helps.