There's a lot of people that also don't know how to write an effective ad. The subject line should be specific, but detailed enough to know what the object is. It should also include the price. If you are selling a car, don't just write "Car for sale". Tell us what model and year and a price. The details of the vehicle can be (and need to be) in the body of the ad.
Take a picture. A picture is worth a thousand words, remember? More like a picture is worth a thousand Bumps. Especially for something that's custom or hard to explain.
Also, try to think ahead of some of the questions a potential buyer would have about what you are selling. Vehicle doesn't have a title? Salvage title, etc? Put it in the ad. That way, people will know if it's worth them looking further and don't have to ask. Some people don't care about a salvage title if the price is right, but others do.