Detailing a car will not affect an automobile trade in value to any significant amount. A professionally detailed car makes a great impression, but it is important to remember that a car dealer will look past a detail job at the true condition of the vehicle. Car dealers have professional mechanics that give a prospective trade in a complete inspection. While a detail job may add a a few hundred dollars to the trade in value, it will not make a significant difference. A detail job will most likely result in a higher sale price if you are selling your car privately.
For more info, see Car Value Calculator: How to Use It