Detailing your vehicle before sale is one of the few things that will actually net you more money than you invest when you go to sell your car (https://lifehacker.com/5991706/how-can-i-sell-my-used-car-for-as-much-profit-as-possible). I always recommend our "Standard Full Interior/Exterior" detail and stress the importance of adding an engine detail to that package as well.
When a potential buyer looks under the hood and sees the heart of the vehicle spotless, with no leaking oil or greasy parts, they'll fill with confidence knowing that the vehicle has been well-cared for, and feel better about spending more on your car, rather than others they've been shopping it against.
Other small additions such as headlight restoration can make a big difference in the final selling price as well. Foggy headlights are a safety issue, in addition to being unsightly, and can give a potential buyer negotiating room to try and talk your price down. If you have any rock chips in the driver's side of the windshield, those will also need to be fixed before passing a safety inspection so these additions may be worth considering as well.
Things I don't recommend in order to gain the maximum selling-price vs. money-invested include things like Paint Correction and Paint Coatings. This goes against what I "SHOULD" be telling you, since that's the life blood of our detail operation, but honestly these services are so labor intensive, that unless you're selling a highline vehicle, the money invested usually doesn't translate to an equally or larger amount in the selling price. Also, minor dents and dings, while they can be unsightly, are a case by case basis. If you're vehicle is a 10-year old family hauler, the next owner likely isn't going to be concerned with some dings and scratches. If you're trying to sell a 3-year old Mercedes that is perfect with the exception of one single door dent, it may be worth spending the money to be able to show the vehicle as having absolutely no flaws. Issues like this are something we can help evaluate with an in-person consultation and we can create a game plan together.
Here's an example of a vehicle that came in for a pre-sale-detail. We did a Standard Full Interior/Exterior as well as the recommended engine detail, and the owner opted to have the headlights restored at the same time. Because the paint was badly marred and swirled from years of drive-through car washes, the customer also opted for a recommended 1-Stage Paint Correction. While this didn't eliminate every scratch and swirl, it made the paint dramatically more glossy and added a lot of shine to it, without adding many labor hours to the service ticket. This investment made by the customer created a dramatic difference in the presentation of the vehicle and ensured they ultimately got full value out of their car