Definitely I would want something in writing regarding the age of the battery(s) and any warranty or guarantees that they mention should be in writing as well.
I purchased my first
ECV used from a Craigslist ad, but that was directly from the family that had purchased it originally, so it had never been used as a rental, and it came with a battery and a spare, and all of the original paperwork, etc.
If you have a chance to check it over thoroughly prior to purchase to make sure you are comfortable with its overall condition, I see no reason not to. Any rental that has held up well enough to be sold is probably pretty tough!

I would just check to make sure that nothing is loose, that the tires are in good condition, and as we already discussed, make sure that the battery(s) have been replaced. Just as an FYI, Li-on (Lithium Ion) batteries are much more expensive to replace than SLA (Sealed Lead Acid) batteries. SLA batteries are heavier, and are like the batteries in your car. They require a whole different kind of "care" than Li-on batteries do. When it comes to any electric mobility device, the more weight you add (with heavier batteries, accessories, etc.) the less distance you can travel on a charge, regardless of the type of battery you have.