Craigslist is the #1 place where scams happen. I myself generally stay away from Craigslist. Amazon.com is a better option, and you have better protection than buying something in cash from a stranger on Craigslist.
When buying electronics or phones. Make sure to thoroughly test or take someone with you that knows about electronics very well.
People have been getting scammed with fake Galaxy S4’s on Craigslist. The way to know the difference between the fake Galaxy S4/S3 and the real one is that only the real one can enter into download mode. I discovered this when I tried to root a realistic looking Galaxy S3 with the real Samsung Touchwhiz android interface. Since android is open source, anyone can install android on any compatible device.
When I tried to root the Galaxy S3 for my friend, we discovered it would not go into download mode and upon doing a Google search I came across an XDA Forum talking about how fakes can’t go into download mode. I told her it’s fake and she said she bought it from Craigslist in Florida. Some looser on Craigslist sold her that realistic looking S3 for $400, that is sad.
Here are some links to avoid scams.