Firstoff, look for scretches abd dents on the body.
Secondly, never buy a phone with shattered screen.
Always check camera lens for scratches, if its scratched, it wont be able to take good photos.
Switch on the phone to check if there is any defect in display or not.
Try to run as many apps you can to check if they crash or not.
Always ask for a bill.
Open camera app and click a few test images with both front and back camera.
Run a benchmarking app to see of the internals function well.