re: Craigslist. There are some good bikes out there, but you need to do your research. The internet is your friend. If a bike turns up, check the model name and year on Bikepedia to get a sense of whether it was a good bike when it was new. You still need to check it out in person. I take it as a given that unless the seller proves otherwise, even a well maintained used bike will need a basic tuneup, new brake pads, chain, and tires. This is cheaper if you do the repairs yourself. Not saying this to dissuade you. But you should know that a $150 Craigslist special could wind up costing you almost as much as a new bike, if you have to pay a bike shop for repairs.
This is is why you need to know what you have. A bike that was Totl when new might be worth repairing, entry level, maybe not.