[Geronimo '74]

All true, I'm sure.

Does that mean that Sid's helmet is crap?? Dunno bout that. If a helmet would pass a thorough visual inspection by myself, I would not hesitate to use it.
Horror stories about people running out of luck because of a bad used helmet are, to say the least, rare.
Who here buys a new helmet every time after dropping it? After knocking it against something when holding it in your hand?
They are made to be sturdy, and can take serious abuse before becoming unfit for use. Issues or defects that would render it unsafe will most likely be visible during inspection.
Manufacturer's recommendation will of course have you buying a new one as fast as possible.

I agree that you need to use good protection for your head. I do not agree that a second hand helmet is almost certainly bad. It just warrants some common sense and a decent inspection.