Make sure the battery is charging/has a charge, I think 12 volts and over?
They run out of juice, they falter and stop.
Pull a plug, if it's wet, I bet he sold it because it isn't charging.
With a fresh charge, that bike will run almost all day on battery before it dies.
get an inexpensive charger, charge it, see what happens........
Welcome to the madness my friend.