Wow, so much to think about. First of all be sure you see it fly and if you can convince the seller fly it yourself. Make sure it responds as it should, assuming you know how it should fly. Check to see it had the latest firmware installed. Look for the dreaded cracks on the arms, under the motors, all sides. Make sure the batteries are all real DJI brand( I would ). Look for signs of previous repair, see if it looks like the body has been opened and if so, why? See if the RTH function works when you test fly it. Have a laptop or some type of unit that can read the sd card from the camera and see if the camera is recording the video to the card. Check the gimbal for signs of damage along with the camera. Shake the remote and listen for loose objects in it, I heard and seen my own RTH button come loose. I'm sure others will chime in with more. Good luck and hope you get a good one.