A couple of years ago I bought an Icon Scorpion helmet off of eBay and it went well. Helmet was brand new in the box with all the stickers, key chains, etc. Shipping and service was awesome... not to mention I paid $200 for the helmet (including the shipping) that would have set me back another hundred bux if I woulda gone to Dennis Kirk.
A couple things to think about though, as some people have alerady outlined. Make sure you're 110% sure of the size because it may not be so easy to return for a refund or exchange. I noticed that D-Kirk's catalog has decent sizing charts for comparison between mfgr's. And just make sure the eBay seller/store has got his stuff together if you know what I mean. I wouldn't buy anything like a helmet from a "rookie".

 

Yeah...DEFINITELY know what size you need/want.

Go to a dealer or local store and find the correct one you want and then buy it off ebay....if it's cheaper

And buy from a reputable seller with a good feedback rating!