I would be more convinced that it only needs a battery if seller dropped a battery in and took it for a pass or two across the yard which is in dire need of a clipping. Do they require a special battery or any 12V battery to make hay.
Yeah...it seems all electric mowers and tractors "ran when parked"! =) Most of the GEs had some open frame relays that were bad for not making contact after sitting a while. The enclosed contactors are typically good. The JDs run all enclosed contactors. The JD mowers I've worked on with this setup are very reliable. They take normal deep cycle marine batteries you can buy just about anywhere. Aside from bearings, motor parts are a little scarce, but are out there. Mostly used parts though.