Oh, there is one thing that I have to tell you that I would never buy used, because it is unsafe: a helmet with styrofoam lining.
You can often see used bicycle helmets, motorcycle helmets, riding helmets, hockey helmets etc. at flea markets, but that is something which you must not buy used. First of all, if it has been in an accident, it is ruined, and you can not always see if it has. Second, impact foam deteriorates over time - it has a life-span of about five years before the foam lining has lost its impact-absorbing qualities and the helmet has to be replaced.
The only exceptions would be helmets only intended for fancy dress costumes (Halloween, cosplay etc.). Hard hats are fine, because they work in a different way.