Forum Moderators: phranque
While there are many free scripts out there and even more off the shelf products that can be purchaced, it all depends on what you are looking to do.
Generally, developing an app like this from the ground up allows you to build it to the clients exact needs and specifications. Having said this, many of the off-the-shelf products and free scripts are very versatile and powerful, although many, AFAIK, only allow for a limited amount of products.
It's horses for courses really - the answer to your questions depends very much on what you and/or your client want. I prefer to build from scratch (using ASP and either a SQL or MDB database).
The best thing to do may be to contact a couple of financial intitutions and see what they have to offer. I'm sure that some other UK based members will be along to offer adice on this one. Different credit card verification and processing services may have different ways of doing things. From your server, you would send the credit card details (securely!) to their servers for verification. Once you receive that you can then tell your site's user that their transaction has been verified and that their product(s) are on their way to them. That's very simplified, but that's how it works here!