Buying Scam
Scammer offers to buy your car, overpaying you for other services, such as transport or shipping, then asking you to pay back the difference.
The SCAM is the payback!
You can lose thousands of dollars with counterfeit cashiers checks or money orders.

Selling Scam
Ad is placed on website for nice car priced low.  Usually no telephone number.  Scammer claims must sell car.  Car is with a shipping company and send them 50% deposit for delivery.
The SCAM is the shipping company is only a phony website in Nigeria, Africa.