Originally Posted by koffee
You really need to ride a bike for proper fitment.
I really don't entirely agree. I've purchased several
used mountain bikes and frames online over the years without any problems (Santa Cruz Blur, Specialized Stumpjumper FSR, Specialized Demo 9, Specialized Enduro SX, Rocky Mountain ETSX, Turner DHR, Giant VT, Norco DH, Karpiel Disco Volante, to name a few). I generally ride large XC frames and medium DH frames. It's easy to tweak the geometry to your liking using different stems, seatposts, cranks, forks, headset spacers, etc... Like most lifelong avid cyclists, I've got bins full of bike parts lying around.
Road bikes are tougher to buy used online. I bought my Lemond Tete de Course based on very detailed geometry figures, but it still took a while to get the bike to feel right.