It works like this…
Apps uploaded/published to Google Play must use Google’s in-app purchase system.
Apps uploaded/published to the Nook app store must use the Fortumo in-app purchase system.
When you build your app in Corona, the Android build setup dialog gives you a list of stores to target. This is the store that you will be uploading your app to and you’re expected to use that store’s in-app purchase system. Your app can determine which store it is targeting at runtime by calling the system.getInfo(“targetAppStore”) function.
http://docs.coronalabs.com/daily/api/library/system/getInfo.html#targetappstore
The store name returned by the above function gives your app the clue it needs on what store specific things it needs to do, such as loading the fortumo plugin or the upcoming amazon in-app purchase plugin. It also makes the “store.availableStores” Lua property obsolete.