Don’t buy it in the dark.
In all seriousness though, you should never view a car you’re buying when it’s dark. There are certain things that can only be picked up in daylight so make sure its bright when you’re viewing cars.
And though some people may be genuine, if they tell you they can only let you view a car in the evening after work, when it’s dark, be wary.
If you’re really keen, offer to view it at lunchtime, or in the morning. If they’re reluctant to be flexible, it’s probably not a good sign.