The Sphere.Io Node.js SDK by Commercetools allows API communication to implement eCommerce features into applications. Additional components include marketing tools, point of sale, and couch-commerce. Developers can access via OAuth2.
MyBantu is an interesting little mashup, available as an iPhone app, an Android app, and a website. Its goal is to make shopping for anything, especially restaurants and movies, easier and more social. It uses Yelp API, Netflix API and various others to make it happen. It's very similar in idea to Siri, the "ultimate mashup" that Apple later gobbled up in the largest mashup acquisition ever.
Although you may not realize it on the surface, DevHub, the free site builder and hosting platform, is effectively a very sophisticated mashup using over 40 different external APIs. The Seattle-based startup, now with 10,000 site publishers who've built over 100,000 sites, have recently launched a major upgrade with new site editing tools, more content (API) partners, and new monetization tools. Their core offering takes data from APIs and other content sources as a way for niche publishers quickly create content-rich sites along with built-in monetization options: