On the page where you submit your ad, there is a "TIP" under the description box that encourages you to make your description at least 100 words long. There is a reason for that.
If you are going to go through the trouble of placing an ad in the first place, you probably want as many people as possible to find it. Making your description 100 to 200 words long will help do that. Here is why.
Every one of the cars for sale, on this site, is displayed on it's own separate page. That page is indexable by the major search engines. That means the search engines, like Google, MSN Search, Yahoo and some others will find the page with your car on it and include that information in their data base. Then when someone does a search, on one of those search engines, for car information that matches your car, they will be sent to your ad.
Those major search sites prefer pages that contain a reasonable amount of text. If you make your description at least 100 words long the page will be seen much more often than it will with a shorter description.
So describe everything about your car you can think of even if you think potential buyers aren't interested. Google is, and you never know what other people might be looking for.
It's okay to capitalize certain words but DON'T USE UPPER CASE LIKE THIS on whole words. Maybe that helped attract the search engines years ago but it doesn't now and it makes the text harder to read.
Don't use abbreviations. Spell everything out. Instead of AC use air conditioner. Instead of AT say automatic transmission.
Submit a picture of your car if at all possible. First because people want to see what they are buying and second because more people will find your ad if it has a picture. The major search engines have a separate data base just for pictures and a very high number of people search for images of cars these days. The image robots (mostly google) crawl this site almost every day.
Good Luck!