Unfortunately a used drone is felt by many to be a crap shoot, due to all that is known that can happen. I would try to clear up as much of the unknowns in the listing. For example; how many flights are on it? How many on the batteries? If the Batteries have not been maintained properly, they're about as worthless as a half eaten cheese burger. Any Crashes - if not say so. If it has a record of several hundred worry free flights - say so. If it has crashed and been repaired - say so and show that it has worked perfectly ever since. Also condition, if it is in good or great condition that may help.
And beyond that there is the reality of what someone will pay for the possible unknowns. For instance I can get a brand new Typhoon H - with warranty and a fresh battery for the aircraft and for the controller for 700 Dollars, add in a good aftermarket battery for 60.00 bucks and I am off and flying. What would a fair price be for a used one where you may or may not need to buy two new batteries? I am faced with this right now as my Typhoon H is been placed into back up position behind my H Plus and if I want to sell it, I would think 400.00 might be the price but then again maybe not. In the end as a backup I can't replace it for that so I will sit on it for now.
In the end - a little more than half of a new base model will be about right. I have a Phantom that I sunk a lot of cash into years ago. When I went to sell it, that number was always in my head so I think I was asking too much. Still have that drone.