craigslist is very hit and miss. It varies a lot by location. I used to live in Southern California and CL was amazing for bikes. Here in coastal Central Florida, it's very slim picking indeed. What I see here is mostly rusted WallyWorld single-speed beach cruisers. Absolute junk. One out of a hundred bikes is worth the effort.

Buying a used bike as a self-professed noobie is fraught with danger. One way is to set a budget of less than half of what you spend on a new bike and realize that this may be a learning experience, rather than the final bike of your dreams. If you learn the right things (how it fits, bike condition, frame style, maintenance requirement, how much you use it) from a $100 bike, that might be money very well spent, even if it is not a keeper. You could get lucky and find your unicorn (it happens), but just don't depend on it.