A couple of things -

1. Like the guys mentioned before - shipping. Some sellers will ship a bike for $25 and some want $75. For most fixies, I can't see spending big money to ship a bike. I guess if you're dropping $1000 on a cinelli or something, that's okay, but for most frames, $75 is huge.

2. Be sure to get detailed measurements from the seller. Explain to them how to measure the seat tube and top tube so that you are getting the bike you think you're getting.