I am selling my phantom 4 tonight. Do I need to clear it or anything to remove any GPS data so the person doesn't know where my house is etc. is there a master rest like when you are selling a phone? Do I even need to reset anything or is the only data saved on the DJI app on my phone and nothing is saved/stored on the phantom?

Just remove or reformat the sd card, I don't think any more of your info is stored anywhere. remove your faa registration numbers if they were installed.

Unfortunately there is no reset to factory specs (like I think there should be).

The person you are selling it to will have to keep your Email address and the password to that email address account, I don't think you can reset a new sign in email to a DJI drone. You Can change the Name of the drone from what you've named it but you can't change the Activation email that you used to activate the bird  You can do a factory reset through the DJI Assistant 2 but that will not erase your activation Email used to set up the bird. That is there till the bird dies.

I purchased used P3Pro and it works fine with my DJI account. DJI has record of activation with your email, that's what starts the warranty countdown. I don't think there is any reason for you to give your email and password to new owner unless you want him to have your flight info. Let the new owner register and start his own account to fly his aircraft.