[munter]

My wife is an ex-biker - used to ride Harleys - she informs me that no-one should ever buy second hand helmets in case they have been use in a crash ( apparently they don't always show damage) No guarantee they are still to spec! It's your life :-)

Most definitely correct, Gordon....however sometimes helmets that are new/unused are offered for sale. 
I am currently watching a Shoei NXR2 that normally retails around $NZ1000 has a starting price of $NZ200 (probably not reserve though) and described as an unwanted prize.

It is often jackets and trousers that get listed with no sizing stated.