Mention whatever it was that made you buy the car in the first place. You won't be alone in your taste. You don't want your ad to describe a vehicle that doesn't exist or give the reader any feeling that you're withholding information. List the make, year and model followed by a description of the car. Conclude with price and contact information.
People often assume an ad without a price means the price is too high.
Check blue book values and other classifieds to figure out what you can expect for your vehicle. Abbreviating saves limited space, but avoid overkill. State your vehicle's mileage only if it is under 10,000 miles per year. For an economy car, list miles per gallon, including city vs. highway. Mention them, if you've saved them.