I know what hat I like and what size fits well so I could buy on line if I wanted - a J3 size2. But I would never buy on line as I would never trust the post not to drop it. I would prefer to travel to the shop to collect it so I know it hasn't been thrown around a post office van!!
I wouldn't buy online. Hats may be the same size on the label, but you will find they all vary in real life. A hat is such a major purchase, safety wise, that you would be far better off buying after consulting a qualified hat fitter.
I have bought a hat online BUT had to send it back three times because of the fit. Ended up spending the money i would of saved on postage and in the end i asked for a refund....would never buy online again, as ive since found out my head is kinda strangely shaped!
I have bought 2 online, one i tried on in the shop, the came home and got it off ebay(how naughty!) and the other was an ajustable, lightwieght type one allowed for driving, but probably not up to much of a standard!
I got one from Robinsons on line as a present from my mum. Fortunately i live near enough to go and change it as it was a bit too small. I've had enough hats now that i think I'd be happy deciding if it fitted or not without going to a tackshop.
I wouldn't buy a second hand one though, or one from Ebay.