Hi
It was possible to set a code to these units and some members have reported “sec” meaning security appearing on the display whe they have changed the battery.
I would confirm with the seller if the unit has a code or not and ensure he will take it back if it does and is inoperable.
some members have reported that the master code is the last 4 digits of the vin number but I’m not sure this is verified.
many years ago I used car audio repairs to repair my backlight costing £85 with return postage - you can google them
hope this helps