Hi Bella,
You're getting lots of good advice here.
You CAN get a bike on Craigslist - that's how I got my first road bike - but if you can, take it for a good long ride first. A good seller will let you ride the bike for at least a half hour. Bring a tool to raise or lower the seat as comfortable (or if the bike has a quick release seatpost, use that.) The saddle should be set so that when your foot is in the down pedal at six o'clock, there is a slight bend to the knee while seated on the saddle. If you can wear bike shorts to your test ride, do.

Also, like RR said, size is different on different models and frames. I ride a 49cm cross style tourer that is a little small for me but is very aggressive and fun to ride, while my road bike is a 54. I am 5'6 with very short legs and a long torso, so women's specific design (even though I have it on my road bike) is not the best geometry for me.

The only way you'll know is to take the bike out and ride it. The bike shop's horse isn't going to tell you if you'll love the ride.

Enjoy the shopping!