I am considering buying a used cellular iPad for my mother on craigslist or locally in general. I would like to get 128GB model, either Air 1 or Air 2. It will be used on T-Mobile network. My only concern buying one is to how avoid buying something that perhaps might be to still on a monthly payment plan or not fully paid off. Can a carrier like T-Mobile or perhaps Apple help me with checking the serial numbers or IMIE if the device is paid off, so I do not run into any surprises? I appreciate any assistance with this.