When thinking of buying used, I'd like to encourage people to check around their city and see if there is a bicycle recycling program available. Most of them have a bike work-off program where if you help them build a bike or two, you get to build one for yourself for free or very cheap. That's how I got my first bike and I didn't know ANYTHING about bikes going in. They helped me, but didn't do it for me.. Sure, it's not the best bike in the world, but when I think about all that I learned while building it myself, I now treasure it.

I also stop by bike shops in the area to see if they have anything they want to donate - that helped me get first dibs on the bianchi frame I built my fix up from.
Most major cities have one available. Free use of their tools and knowledge!
Vincent