If you crash a quad badly its pretty hard to hide the damage.
If the thing flies straight, looks straight and behaves as it should, there's no reason or logic to suggest anything is wrong, if it there is something wrong, it should be evident.Even if it had been crashed, you would not scrap a car if you put a dent in the door, replace the door and the car is fine. SO even if it had a bump, not the end of the world.
The only thing I'm really fussy with is batteries, I prefer to buy new.
I sold my phantom 2 last year, I can honestly say it was never crashed once and I would buy it back in a heartbeat, I just needed the money at the time to fund my X5.
As always look at the person flying it, do they seem like an idiot, do they fly so well when they demo it you don't think they would ever crash?
its nice to buy new, but there are some horror stories of drones and batteries failing in flight from new.
At least when a drone is semi used you know its reliable/ISH.