sales inventory system

I'm trying to build my own little web site - an online store in ASP/MS Acess
First of all please let me know if someone has some sample application of this kind.

As I'm trying to write my own code I'm particularly interested in knowing how do we do:

1) e-mail verification - how do we know if that e-mail actually exists
2) credit card verification - How do we know that the customer's
credit card# is valid etc. Is it done in code or a third party does it.

Please suggest

For examples you could look at www.planetsourcecode.com

Regarding accepting credit payments as far as I know a third party does it, in the UK you need a Merchant ID to accept payments into your bank account.

Another alternative is to use a third party such as Paypal or Worldpay.