PV, This is the ebay receipt for mine. Mine is (Verizon) 4G Lite or CDMA. The other (AT&T) will be a GSM phone. You need the one that matches your carrier. You want one with a clear ESN. That is the info that allows it to be placed on your network as a clear to use phone.
There are a number of ebay sellers. The iPhone 5 is the least expensive phone that works with the app. Some sellers have bought up used ones from carriers and will be phones that won’t be denied by another carrier. I’d message them to make sure they will work with your carrier without problem. That’ll give you some eBay protection, if there is a problem.