The Google Shopping Content Objective-C Library by Google can access API data to sell products on Google, list ads, drive traffic, and increase sales. Developers can interact with the JSON API, working with iOS 3+ and Mac OS X 10.5+. Available introduction and example applications.
Blippy and Swipely both launched fairly recently with the hopes of making spending a social experience. Both services are very similar in that they both allow you to connect your bank and credit card info, as well as other accounts like iTunes and Amazon. After you've set everything up, as you start spending, you have the option to share these purchases with everyone. And now you can extend that sharing with the Blippy API.