It's a common scam. They give you a counterfeit check or MO for more than your price and then ask you to send the excess back to them (with your legit check or MO). Some of the checks are darn real and so are cashed by the bank; however in a few days when they discover the fraud, they pull the money back out of your account.

This scam is so old, yet it must still work quite well.

-gmt