"The Snell Memorial Foundation has tested motorcycle helmets held in storage for more than 20 years and found that they still meet the original standard. EPS is a long-lived material little affected by normal environmental factors. Unless you mistreat it we would not expect it to "dry out" enough to alter its performance for many years."
I came upon this thread through Google because I found a helmet importer selling some New Old Stock and wanted some info. They are clearing out their warehouse of some European helmets that are no longer imported to the US. These helmets are 10 or so years old and were $300 when new. They are selling them for #35 with free shipping.
Also, some say never to buy used, but there are always extenuating circumstances that make it a good choice. How many guys bought their wives full gear only to have them refuse to get on a bike after their MSF course. Ho wmany geezers thought they could relive their youth only that what was fun then might not be fun now. Their are also those like me who buy online and then find the thing is too big, to round or whatever after wearing it for a couple of days.