I have been through this before. Basically they send you a fake western union or some other money check and want you to cash it. I got one for like triple the asking price of the item I was selling. They said something like my assistant made a mistake and sent the wrong amount. They then want you to cash it and keep the money for the item, and even some extra, and send the rest back to them with the item. My hunch is that sometimes they can get cashed and other times the check cashier employee raises the alarm and then you are stuck with the issue of trying to pass a fraudulent cashier check.