NEVER BUY A USED HELMET!!!
ALWAYS buy new. ALWAYS.
You have no idea what that helmet's been through, nor do you know how old it is. I'd never take a chance on a used helmet.
There are approved helmets out there that are less than $50.00 new, so there's no reason to cheap out and buy a used one.
If you think saving money is more important than your brain, your priorities are skewed. Badly.