As mentioned above, if it hasn't been started regularly (monthly even) seals and gaskets that are expected to get lubed and wet dry up and can fail once under pressure when it's are moving.
I had to pass on a great deal I've because of that. Hadn't been started in 10 years. And yes do not ask them directly if they started it for those reasons. Just find out in conversation how often they used it, the last time, and so forth. You'll get a better idea of actual use.