My first bike was purchased at a LBS. I knew nothing about bikes and it was the best purchase I made. That folding bike has become my beater bike and most of my knowledge came from that cycle.
I've also purchased a bike from Ebay but this was basically a beater that I sold after several months. I would never purchase a $1,500.00 dollar bike from Ebay. Never.
In fact, if you're going to spend more than $700.00 dollars on a bike, you're better off buying new. Buying a used bike means giving up warranty repairs and fee adjustments. If you don't know how to fix or tune a bike or what size would fit you, BUY NEW!