Battery health is most important since a battery is expensive to replace. It's hard to judge battery health but age and number of miles is a proxy. IMO 3 years old or newer and less than 1000 miles is almost new. But the best way is to test drive a bike at fuil power for a bit to make sure it responds and doesn't lose capacity quickly.

If you are in the US upway.co is a place to buy low mileage ebikes and they come with a one year warranty. A brand that has a brick and mortar store near you is optimal since eventually you'll need service and shops tend to not want to work on the electrical bits of bikes they don't sell.