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.