Swappa is an online marketplace where you can buy and sell gently used smartphones, tablets, and smart watches. It sort of makes sense it would have an app, but they only just now got around to it. It doesn't really do a whole lot right now, though.
The Swappa app is really just a price checker in this initial release. When you open it, the app identifies your phone and gives you pricing history, as well as what it's going for now. There's also a search feature so you can check other devices. At the bottom of the listings for each device you have buy and sell buttons, but all they do is open up the Swappa website in your browser.