Clean the engine
Most modern cars have plastic covers over the engine. These surfaces gather grease and road dirt and can look pretty nasty. You can use a spray detailer or any cleaner. This is a good time to check the fluids too, such as oil and coolant levels.
Blacken the tires
Tire black or tire shine is sold by many car care companies. Cleaning off the brake dust and putting some kind of treatment on the tires will sharpen up the look of your car more than anything else you can do. And take a rag to the rims before you spruce up the tires. Especially with the front wheels, brake dust will coat surfaces that should be shiny.
Touch up glaring scratches
If there is an ugly scratch in a noticeable place, it might be easier to fix than you think. Sometimes, the scratch is only in the clearcoat, the protective outer paint layer that is basically clear plastic. If you get a clearcoat pen, you can make the scratch nearly invisible.
Give it the curb appeal test
Now, clear your mind and look at your used car with fresh eyes. What jumps out at you? Is there anything else that needs to be touched up or cleaned? Or does it look so good you are going to raise your asking price?
There are a number of other things you need to do to sell your car, including locating the title and having the service records ready to show the buyer. A complete description of the process can be found in "10 Steps to Selling Your Car." However, the items listed above are specifically for producing a positive first impression. After all, you want their very first thought when they see your car to be: "Nice!"