First and for most, if your going to buy anything from a facebook fleamarket or from craigslist, go to a safe location. Some cities will allow you to go to your local police department to make your transaction.
1) Don't buy tickets from facebook or craigslist groups. The reason I strongly suggest against it is that there really is not a way to check if the tickets were purchased legally or if they are just fakes.
2) Electronics should not be purchased unless you test the dam thing. What could be worst is going home only to find out the electronic you purchased didnt work.
3) Cell phone. This one is the worst you can buy online. You take a real crap shot if you but one from an online group. Sometimes these are stolen phones which are locked and you end up with a brick for a phone that does not work.