The value of a used motorcycle helmet is exactly $0.00. And isn't it funny how every single used motorcycle, dirt bike, auto-x or bicycle helmet is magically "never dropped, never crashed, only a few miles on it, etc."
There's a great sign hanging up in my LMC (Local Motorcycle Shop): "If you have a $2.00 brain, then buy this $2.00 used helmet"
Would you buy a used car seat for your kids? I wouldn't. Precisely because despite what the seller says and no matter how trustworthy they are - you never know the history.
Read this article (very good information):
http://www.motorcyclistonline.com/gearbox/motorcycle_helmet_review/index.html One of the very best helmets out there is the Z1R-ZRP-1, which can be had for well under $100 brand new in the box. Unfortunately it's made in China, but when it comes to protecting your skull and its contents, I don't really care where it's made - I care about getting the best protection.