Simple maintenance checks and repairs to help sell a car on craigslist or ebay. It's very important to ensure that your car is in as good of shape as possible prior to selling it. 
This is because easy repairs such as valve, maf cleaning and oil changes are very simple to do, however they can drive up the price of your vehicle and increase the chance that anyone who see's the car will purchase it.

Every time I check a car to purchase on Craigslist, I check if there is a check engine light, and then I check the oil, coolant, and idle. If the oil is dark and/or has a burning smell, I immediately take it as that the car hasn't been treated well.

Avoid these mistakes by checking the fluids in your vehicle, and getting rid of a bad idle/acceleration.
Ensuring that the coolant is at the normal level and normal color is important as well.

To many sellers worry more about the look of the car such as the paint, tires, and/or interior, but pay little attention to how the car is actually running.