I too, one day would like to sell my business. But recently my wife purchased an existing service based business so I have recent experience with this from the other end.
When we purchased it, we purchased it based on what it was at the moment. "Potential", I dont feel it necessarily has any pricing merit unless its something new and proprietary, or the location (not for a mobile guy) All potential comes from the new owner working hard for it, and in that case he shouldnt have to pay for it. In a service based business, results rely on the person behind the wheel.. once the driver changes, so do the results. SO future repeat customers aren't necessarily guaranteed.
For my wifes business we purchased: Web site (1st page/first search result on google, yelp, bing, and yahoo), yelp & google pages with multiple positive reviews, business name and location established 2 years ago, client list with email addresses & phone numbers, and more. But we purchased with no equipment as we had our own. There was some extra "potential" value there based on the location, but we didnt factor it into the negotiations.
Honestly I think it'll be hard to sell a small mobile detailing business for any decent money other than what the equipment is worth used. Anyone can just make a logo, goto checkers or walmart and buy supplies and advert on craigslist to get started. Thats basically what I did. Now I have a fixed location with 5 employees.