It may depend on what standard of helmet as to what's expected and buying a second hand one isn't something I'd really entertain anyway but I'd be livid if I got one delivered just wrapped up. Box with polystyrene inserts to stop it rattling would be what I'd expect.
I've sold a helmet on ebay, what sounds like an acceptable level of packaging?
it has a bag, can i then just wrap it up? its obviously pretty solid.