Generally, so long as the frame size is right, you'll be able to make the bike fit you. No bike will be a perfect fit out of the box, one advantage to buying from a shop is they can fit the bike to you, more or less. One thing you cannot know when you buy a bike online are its riding characteristics and its handling. Steeper angles and a shorter wheelbase mean sharper handling at the tradeoff of slow speed stability and vice versa. Geometry, construction material, and the dimensions of that material have their own characteristics. It's not necessarily that important, most people tend to adjust to the characteristics of the bike they buy rather than try to find a bike with the specific characteristics they think they want. I don't mind buying bikes, new or used, off the internet.