I just purchased my first bike, Being first new buyer, I've felt the overwhelming experiences.
I believe that buying bike is like buying clothes or shoes, that fittings is very important. Hence in future after I've tried hundreds of bikes and brands, only then when I might be able to know if buying from Ebay is a good way, talking about price Vs fitting.
What I did was asking lots of questions in the forum, looking at websites to find info about the bikes, visiting LBS with some ideas already inside my head, so eventhough the LBS person has his own idea, at least I can back up with the info I've gathered.
Another thing, even with buying through LBS, one is supposed to visit various LBS to have a feel on the service they get, so it's not just buying bike that i have to shop around, I also do the same thing with choosing LBS.
Buying through Ebay, for first timer, I'll say if you can come to the seller's house and try the bike, then you'll know whether to buy through Ebay or not.
To sum up, I think to approach both internet and LBS are good way, so you'll have many info and at the same time try the real bike.