Your client needs a merchant bank account that accepts
Visa/MasterCard. Next, you need an account with a service called a
payment gateway, such as Authorize.net, Verisign, etc.
Once these are set up, you need create a form where you ask
the user to submit card number, name on card, expiration date, etc.
Once you have that information, you need to do a CFHTTP to the
payment gateway URL and submit you data to it. You will receive a
response in CFHTTP.FileContent that tells you if the payment was
accepted. If so, display a receipt. If not, display an error
message to the user.
You can learn more at
http://www.authorize.net,
http://www.verisign.com/ and
http://livedocs.macromedia.com/coldfusion/5.0/CFML_Reference/Tags47.htm#1632966