Hi there and welcome to the Inspire Pilots forum. I'm not sure if you're aware but there is also a dedicated Mavic Pilots forum here: DJI Mavic Drone Forum
Gain as much information as possible from the current owner in regards to flight time, number of battery cycles, place of purchase (valid warranty). Also be sure you're protected with your method of payment if it does not turn out to be what it is stated to be.
Once a unit is registered to a DJI account it cannot be re-registered to a new name or account (not all that big of a deal).