It seems there are a lot of scamers. Here's how some of them work:

read full article here: http://a.wholelottanothing.org/2008/03/03/a-used-car-scam-i-once-almost-fell-for/

1. A guy called within an hour of posting the for sale notice.
2. He had a sob story about his sister that needed a car very badly to get to school, and how he’s ready to buy one for his sister asap.
3. He showed up with a friend who he said knew a lot about cars. They both rode along for the test drive.
4. We stopped at a supermarket parking lot after driving a bit, and the mechanic friend gave the car a once over, looking under the hood and looking around the entire car
5. Mechanic friend comes up from the tailpipe with oil all over his fingers, says the engine block must be cracked, sending oil through the system.
6. Guy sounds kind of bummed about the “cracked engine block” and we drive back to my place where I drop them off.
7. Before he leaves, guy offers $1,000 for the car, since his sister still really needs to get to school and his mechanic friend promises to fix the engine at a later date for a higher price.
8. I refuse, saying it’s a ridiculous price. He offers $1200. I say no thanks, and leave.
9. The guys hang out in front of my building, calling 15 minutes later with an offer of $1300, and waits another 15 minutes to call again with an offer of $1400. I say no both times.
10. Three days later I sell the car for the list price, from someone paying cash and looking to refurbish the car top to bottom.
11. Original guy calls after one week from the same number, using a different name, asking if the car is still for sale.