From my limited experience of helmets, buy from a mate or buy new. Helmets can be seriously degraded by a previous impact, and it is very difficult to tell if it has had such damage.
That said, this is risk assessment on your part. It will be good enough to stop you banging your head against the tail skid on the walkaround, but in the unlikely event that you need that 9G protection, it may not be enough.
You get what you pay for, and there is no free lunch.