The camera resolution sounds more like that of an iPhone 6 or 6 Plus, than 6s and 6s Plus. All of these iPhones have Touch ID, though. Maybe they have been repaired by someone else but Apple, and this feature it's not working any more.
If these phones are cheap, it's also possible that they are still connected to an iCloud account, and can't be used.
No idea whether or not they are fake, but if it sounds too good to be true, there's certainly something wrong.