Should I buy this (gently) used, 15 year old, helmet? My neighbor is moving cross country and selling all his bike stuff. His helmet's a white Bell, about 15 years old. It's a bit scuffed but generally in pretty good condition. The only thing that worries me is some of the styrofoam is cracking off, but it shouldn't be hard to glue or remove and stuff a sock in the hole. And  the right side is a bit deformed inward from storage (he says) but it bends back when on my head. But the straps are fine and it buckles great, plus it still looks stylish after all these years. He's selling it for $10, which sounds like a steal. Should I snag it for cheap or splurge on a new one?
No. It is safety gear – buy a new one.

The old helmet might make a good pot for a hanging plant but it is more likely to fail dramatically in your moment of need.

The straps would probably come right with a soak in water, but the polystychrene is the functional part and it has failed with age.  

It might fill a legal requirement if your location mandates helmet wearing, but it probably won't provide sufficient protection.