My favorite was a neighbor who wanted to sell something on CL. She asked me to take some photos of the item for her, which I did. She then asked if I could insert them into a response to an email she had gotten about the item. While doing so, I saw it was a classic 419 scam. So we set out on own little sting operation.
She replied, and sure enough, was told she'd get a cashiers check for $1200. The item sold was for $800.... the remaining 400 was for 'her trouble'. A few days later, the check arrived in the mail, with instructions to deposit it. Of course, she didn't. Instead, she called the law. And a week later, she got a call that 'an associate' of the buyer was in the area to pick the item up. That's when we sprung our trap.
It was a large item, and she was going to call me to help load the item into the associates' truck. But before I went over, I had already formed a text message to the sheriff's deputy in my area on my phone. As soon as I got the call, I hit "Send". A few minutes later, there's a sheriff's patrol car in the street, blocking the 'associates' car from leaving (we live on a cul-de-sac). The poor sap never saw it coming. Fake cashiers check, fraud, illegal use of a financial instrument, the whole nine yards. The fake cashiers check ended up in as evidence, as did all the emails.
It was fun as h-e-double toothpicks watching him get cuffed and stuffed.