Moral of the story don't buy a used drone. Specially without a controller, and without physically examining it and seeing it fly.Major problem with FACTORY RESET. if the firmware is version **.**.0770 or **.**.0790, the firmware has anti-rollback version #2. No-one has managed to find a way round it yet and this means you can only refresh the current firmware, you cannot install an earlier version and it won't let you return the drone to original factory settings.
Simple question, has the seller unbound the drone from his DJI account? If he hasn't and you're trying to fly what DJI thinks is still HIS drone on YOUR DJI account, you won't be able to use it. Worth asking