If you can ask for the IMEI number of the device you could also call Garmin Support and ask them if there is an active subscription or any other issues with the status of the device.
An inReach subscription is linked to the IMEI number of the device, which is different from the serial number. Garmin Support should be able to tell you if the device is locked (in that case do not buy the unit) or whether there's an active inReach subscription that needs to be cancelled by the owner before selling the device to you.
