Does anyone know much about mobility scooters?

Got a bit of a problem, I need to know whether it is a big job to fix or not.

I have a second hand scooter I was wanting to sell on as OH has just bought a new one.

The battery is fine and power going to machine all showing fully charged, it worked fine yesterday then today a prospective buyer took if for a ride and it just stopped.

We have had a look at it and established that the disengage mechanism seems to be causing a problem and the green light that usually shows continuous if all ok is now flashing which is telling us there is a fault.

Is there anyone that maybe able to give us some clues about it. many thanks.