I know someone who fell for the "send back the extra money" scam and luckily he mentioned it to me in process so that he only got taken for $160. Here's how it worked:
My friend (a regular guy in his 40's) posted two chairs for sale on Craigslist. He was contacted by a guy from California who said he was moving to this city and that his employer - a university - would be paying all of his moving expenses. Problem was that the university could only issue checks in amounts of $500, so when the check from the university arrived would my friend please deposit the check into his back account and send back the $425 difference? My friend deposited the check (it deposited just fine) and was about to head to the post office with his own check of $425 when he ran into me and mentioned where he was going. I'd seen a report of this scam on 60 Minutes, so told him not to do it and immediately contact his own bank. Well, his own bank charged him $160 in fees for depositing a bad check, stopping payment, putting a hold on his account and who knows what else.
I think the problem is that if you're an honest person you have trouble seeing that other people are not as honest as you are.