I have bought a ton of stuff on Ebay and have had good luck. BUT, do your homework first. There are just as many scams on their as their are good deals. I stay away from used stuff and ask alot of questions BEFORE the sale is over. The biggest one is ask about shipping and handling before the end of the auction. That is the one area that seems to be taken advantage of the most. I use the ask a seller question for anything that pops into my head. Some of them don't respond before the sale, so what do you expect after the sale.
Just do your homework and know exactly what you are buying really costs. Some dealers advertise on there for more than you can buy mail order. I looked at the ad you posted and two BIG RED FLAGS come up. First, they have a zero feedback rating. They could be new on Ebay or they could be a scam artist that just switched their screen name. I would ask them about that first. Second, they have not posted an actual picture of the helmet, that is a photo from an ad. Click on "ask seller a question" and ask them for an actual photo of the helmet. I would ask in that same email about their feedback. If that is a new helmet as the ad says find out the name of their dealership or shop.
There may be a very good reason why nobody has bid on this helmet.