I've experienced a few apps that have had some time on the Mac App Store, but were later removed either by Apple or the developer for whatever reason. The developer then followed up with an offer where you could register the app by downloading one from their site onto your system with the App Store version already installed, which would unlock the full features of the new download.
How does one go about verifying an authentic purchase like that? Ideally I'd prefer examples in Swift if you would be so kind, but ObjC is manageable as well.