They charge a 15%+ premium for their efforts, which I can do all myself. In fact, I do more than what they would offer their clients. I get more peace of mind than what they offer.
Trust me, people buy lemons from dealers too.
I won't keep longer than 150,000km so I can maximise what I get out of the car when selling by taking advantage of the 'lower mileage' myth. I'll get much more, and be in for a much quicker sale, than if I sold at 200,000km.
On a more personal note, my car is extremely rare and desirable. Once I've sold it, I'd have only lost about 5 - 10% from when I bought it several years prior.
If we're working with figures, someone who buys a car new for R300,000 will lose around half of that at 100,000km/once plan has expired. I would then buy at R150,000 (or less), and only stand to lose around 10 - 15% by the time I sell again at around 150,000km.