I think as with anything, you will get mixed results with buying any used product online. Amazon and eBay both have buyer protection and various dispute processes but it does come down to seller against buyer. A drone is a special case because of its delicate nature. What if it was indeed working fine when it was shipped but the gimbal does not work when you receive it? Who is to blame? Who is to be believed? Of course it could be the shipping company but now you have all sorts of finger pointing.
Personally I have only bought and sold drones in person, face to face, with a thorough test flight. I know many have bought used drones with no issues but that is just my personal preference.