Buying a used motorcycle helmet would be like buying an air bag for your car from a junkyard, when the rubber hits the road it may or may not save you in a tight situation. Your life is dependent on this piece of equipment so you don’t want to be cheap where it counts the most - your head!
Since you don’t know how the previous owner handled the helmet you are taking a risk on whether it will provide proper safety for you. Even if the previous owner didn’t have a wreck with it on, they may have dropped it hard enough at one time to create a stress crack in the helmet.