I and my friends have encountered these scams numerous times. The scam actually revolves around Paypal, and not craigslist or whatever the listing / auction site is. They open up a conversation to obtain your email address (that's how craigslist works, unlike eBay which specifically discourages people from communicating outside their system). Once they have your email address they say they're sending money, and *oops* they "overpaid" by accident. They send a spoof "You've Got Money!" Paypal email from some domain that contains "paypal" in the domain in some way. Usually crap like paypal.asdasda.com. That is the key to the whole thing, is that fake Paypal email that makes it look like they paid.
Then they immediately start threatening legal action unless you refund them the amount they overpaid. However, instead of Paypal, they insist the money be sent via Western Union.
That's basically it. They typically copy / paste some background story about them being out of the country or something, and they'll have a transport service come get the vehicle. The "marine biologist on a ship doing research" is a story they were using a lot a few years ago.