While you're at it, try to sell your client on the value of a full tune-up at the same time. Top performance is expected by the buyer, and an honest explanation that this is the way the car runs normally will not cut it at the time of sale. Budget any minor repairs such as muffler work, new shocks or brake bleeding that you think might add to the value of the car, but don't be pushy. Calculate your time at a rate not quite what the Tidy-Car people charge and try to offer more personalized service. This will lead to repeat customers and those all-important referrals.
Some people have specific models of cars on which they can work miracles.
The Beetle genius is probably the best-known, but there are many who work on Camaros, Corvettes, and certain popular Japanese makes. If you are one of these people, you can probably do better work than a mechanic in less time, charge less for it, and build a steady clientele. If you are not a licensed mechanic, you cannot advertise that you are, and when you figure your costs, add a little bit for the follow-up work you'll have to do. It's better to satisfy an angry customer than turn them away and it's easier to reconcile in your own mind when you figure you've already been paid for the extra service. When you advertise your service, keep your prices reasonable compared to a regular mechanic.