Look for "screen burn" (ghostly images) on AMOLED displays and check for dead pixels by viewing a solid white or black image. Inspect the charging port for loose connections.
Many manufacturers have built-in diagnostic menus (e.g., Samsung's *#0*# ) to test speakers, vibration, and sensors. buy used mobile phones
Aim for a device with at least 80% battery capacity . For iPhones, check this in Settings > Battery > Battery Health . For Android, use apps like DevCheck. Look for "screen burn" (ghostly images) on AMOLED