The first warning here is that to truly make the big money on craigslist you need to work at it non-stop and you need transportation (preferably your own vehicle with lots of space) and be able to lift and walk without much effort. To be the most effective you need a sidewalk or lawn to put things on.
I have a friend who simply scours craigslist constantly simply finding good deals and then selling them to someone else. You can sometimes actually sell things being shown on the free section of craigslist in another place, like ebay, consignments stores and antique stores or back on craigslist. My friend will pick up things from the free section and drop them off in front of his yard with a $20 sign on it, and make a new post on craigslist's for sale portion. Amazing how if you have the space and the patience you can sell just about anything. It only takes a few cash-ins per months to be swimming in it. If you are willing to go outside, or have a car you can amplify your earnings by picking up junk and reselling it to consignment shops or antique stores.
The biggest downfall of this strategy to make money is that it is very easy to fill your living space or yard with junk and become a massive hoarder. You need to be detached from these things and if they do not sell after a set amount of time (two weeks?) then you need to get rid of it. The amount of stuff you can collect is endless on craigslist especially if you can drive long distances and live in or close to a city. A rural part of the country it may be much harder to make this profitable.
Dink and Dunk
You might spend 4 hours a day collecting things, and a few hours a day selling things and only make a few dollars here and there. But once you get into a routine it is quite easy to make between 20-60 per day consistently, and some days you might make $200. The key thing is to be a good communicator and have a good eye for what people will buy and for how much they might spend. After enough experience you can spot diamonds in the rough and also be able to spot which things are not worth picking up.