The most common such scam at the moment is where someone offers to buy your item - usually advertised on Craigslist - and sends an authentic-looking cashiers or payroll check, or Moneygram. They are so sophisticated that these even fool bank tellers. The thing is, it will be for a much higher sum than the asking price - often by a thousand dollars or more.
It is always followed by a frantic email, saying something like "my secretary/assistant (etc.) mistakenly sent the check in the wrong amount/sent a check meant for a different purpose. Please deposit it; you can even wait a day for it to clear, but please deduct the asking price, plus another $100 for your trouble, and send the rest back to me by Western Union. The contact identification is [xyz29564 or whatever.] I will arrange for the goods to be picked up." Of course, after several days - possibly as long as a week - the check/Moneygram bounces and your bank account is debited the amount, while your money is gone, gone, gone.
Thousands of people have fallen for this.
It was tried twice on me, in 2011 and 2012. The first was a Moneygram. I was totally unaware of the scam, and could have been duped too, but luckily I'd sold the lawn tractor to someone for cash the day before, so I emailed asking where I should send the Moneygram back to. I heard nothing, but I decided to deposit it anyhow just to see what happened. Sure enough, I got a letter from my bank a week later saying the amount had been deducted: "No effects. Refer to drawer." So it was $ in and the same $ out. A knowledgeable ex-banker neighbor said not to try that again, as you could be penalized or even run into trouble with the law.
Last year I advertised a camcorder on Craigslist, and a check, ostensibly from the City of Detroit, arrived by FedEx for $1450, which was $1,150 more than my asking price for the camcorder. This time I was ready for the frantic email saying that he secretary had mailed a payroll check in error, as the very authentic-looking check was spelled CITY OF DETRIOT. I emailed back, thanking her for allowing me to keep the $100, and saying I had complied with her request by mailing the rest to the address the FedEx came from, in Encino CA, in $100 bills. An even more frantic reply, saying "Go out and stop it from being sent out of the post office! You must use Western Union. I am in London on business." (These bastards use holding addresses, usually in another state. And they themselves are often in another country, like Nigeria.)
My final email was "Don't worry, the $100 bills are good. I know, because I printed them myself, the same way you printed your phoney check. When they arrive, roll them up and stuff them up your *ss."
I did contact the police in Encino, where the FedEx originated, but they said it was a waste of time trying to do anything. The person there would just be paid to send out FedEx envelopes.