And to add to Aaron's comments, (Carrera26) as I understand it helmet standards aren't the same from motorcycles to ATV's to Auto Racing. Because the accidents for each type of vehicle are different, the testing, standards, helmet construction and general usability are different.
I wouldn't typically reccomend buying a motorcycle/atv/snowmobile helmet for use in an automobile but if you are just going to auto-x I guess you just need something around your head for protection. So I guess with that line of thinking, there's nothing wrong with picking up a used helmet. I know that when I was auto-xing last year, the SCCA had "loaner" helmets that the newbs could use on a per run basis. Those "loaner" helmets weren't broken in for the newbs so I can't say that it matters too much in a parking lot.
But beware, odds are that if you want to take your car to a track day at Watkins Glen with an organization like Trackmasters, they won't let you use your motorcycle/atv/snowmobile helmet. So if you're planning on doing more than just the standard cone racing, I'd invest in something Snell rated specifically for Autmobile racing.
2004 ZX5 5spd; pretty stock, and slowly rusting