Thoroughly clean the inside, wash and detail the outside, tires, and rims. The auto parts stores sell an engine wash that can be sprayed on (cool engine of course) and washed off to de-oxidize the metals. It’s not worth putting too much time and money into a car that won’t bring top dollar anyway. Focus on the performance aspect when selling it. If everything is in good working order and it will easily pass inspection , you will get as much as is possible out of it. Mileage has a big impact on what others are willing to pay. Good records of periodic maintenance also help. Good Luck!