since, to your customers, your skill is your primary product that you are selling, and means everything to your customer, I would be very careful about just selling it for a value related to monthly profit, to just anyone that comes along. It seems like you have built up a good customer base, and that was no quick and easy project. Have you considered taking on another mechanic, and if it works out as an employee, giving him a chance to earn a portion of the company each year, (maybe instead of a bonus 5% of the ownership the first year, and 10% of the remainder for any year that exceeds budget expectations). this would give you and your customers a bit of 'insured continuity' with your company.