Your intent is unclear. Are you trying to sell content for your application? If so, there is an established, Apple-approved way to do this:
http://developer.apple.com/iphone/library/documentation/NetworkingInternet/Conceptual/StoreKitGuide/AddingaStoretoYourApplication/AddingaStoretoYourApplication.html
If you're trying to sell something non-iPhone-related, you should redirect to a web page that actually handles the transaction. You can do this as a link that opens a page in Safari, or as an embedded UIWebView
.
The first approach is simplest:
[[UIApplication sharedApplication] openURL:[NSURL URLWithString: @"http://www.google.com"]];
The second a little more complicated:
http://developer.apple.com/iPhone/library/documentation/UIKit/Reference/UIWebView_Class/Reference/Reference.html
If your question is really asking how to build an entire product purchase API, then, not to sound sarcastic, I suggest this site as a useful starting point:
http://www.amazon.com/books/