Do people seriously fall for scams like this? There are two glaring red flags in this scenario that were completely lost on the author: 1) The coincidence of someone finding the necklace and another person phoning in about it while the first guy was still in the store, and 2) the first guy asking for cash.
In my experience, coincidence--or any kind of anomaly, really--and strangers asking for money individually should be enough to arouse suspicion. Together they're a dead giveaway.
I find that you should also be wary of people who accept others regardless of their shady behavior. Usually it indicates they are just using everyone around them, as they don't care about moral values.
There's a huge difference between reading about this on the web and actualy being in an emotional situation where you want to help someone and don't diligently think all the consequences through befor you act. A situation also known as everyday life.
In my experience, coincidence--or any kind of anomaly, really--and strangers asking for money individually should be enough to arouse suspicion. Together they're a dead giveaway.