I work as a auto technician. Been one for 16 years. When I buy a car to sell, people start blowing up my phone. I usually spend about 2 months getting it ready to sell. I work on them on my days off, and I make sure it's a car I'd sell to my mom. I don't do it for the money, I do it because I enjoy working on car on MY TIME!!
I usually sell the car for a little more than its worth, but the people who buy from me know that the car is not going to have any major problems with it down the road.
I sold 2 cars this year, a2003 Acura TL Type-S with 250,000 miles of it for $4,000; and a 2001 Honda Civic for $3,000. In both cars, I did the timing belt, water pump, idler and tensioner pulleys, coolant flush, power steering flush, evac and recharge the A/C system, new fuel ,air and cabin filters, a complete interior cleaning using a hot water extractor and steam cleaner, and a full paint correction, which usually takes about 2 weeks to get it done right.
I sold both cars within 3 days of putting them up for sale.
Perhaps your mechanic friend should invest a little more, and charge a little more for decent rides.