Sorry, but I would never buy a used helmet. No used helmet is a good deal unless you can for sure validate the owner has never dropped it or been in an accident. Some things are worth paying full price for.
From the web (and from Motorcycle courses taken):
Using used helmets is extremely dangerous and impractical. Used helmets are actually worthless and what is more you cannot really tell whether they are damaged or not. You won’t even be able to tell if they are relatively new ones or not. This is because the helmet is made from a hard outer shell that envelops a layer of expanded polystyrene. You will never be able to tell if the expanded polystyrene is already compressed or not unless you cut the outer shell. An already compressed polystyrene layer will not protect you from an accident.