You could ask for a screen showing that the seller unbound it from his/her account. You could ask for a signed affidavit that the seller unbound it from his/her DJI account.
Don't know where your drone is, don't know how to fly it home?
No map needed.
Learn to use the radar display.
Switch it on ...
It shows you the drone is out there in the distance.
Rotate the drone until the drone icon is pointing to home.
Then...