If I remember correctly from selling a plow truck on a local classified site, here's how it went:
Got an email from some foreign national who wanted to give me way more than I was asking for 1978 Chevy K10 w/Fisher plow. But here was the catch: He'd send me a post office money order (which can be faked) or cashier's check or some other such instrument. The victim cashes the instrument, keeps the money for the vehicle and then sends the excess back to the "buyer" so that he can "arrange for shipping". Of course he'll get back to you later with shipping details.
Guess what? You never hear from him again. And you are the one who signed and cashed the fake instrument, making you responsible to make good on the bogus check, MO or whatever with your local financial institution.
It's an old internet scam.