I have been known to buy used. My current racing bike is a used bike. The guy bought it for almost $1000, used it once, then put it in the garage.
HOWEVER I draw the line at helmets. Helmets are designed to protect your brain.
I've taken falls that wound up with a head injury and my helmet had no dings, dents, or other problems to show that it had taken a serious impact with the ground. It protected me from a serious head injury- but showed no signs of it.
On top of that you very rarely are ever buying from someone you know. That said- you have to take their word for it that there are no problems with the helmet. You have to take their word that they haven't abused it or fallen with it.
Saving myself a few hundred dollars compared to the very real possibility of having a head injury that could leave me damaged for life isn't worth it. I had a mild to moderate concussion from a fall- my short term memory was affected permanently. I don't remember things the way I used to. Thank God for the helmet working the way it intended.
Would you risk your brain for $100? I sure wouldn't.
Do what you feel in your heart to be right- for you'll be criticized anyway. You'll be ****ed if you do, and ****ed if you don't. - Eleanor Roosevelt