I am aware of a 23,000 mile 1998 SL500 with panoramic roof and heated seats but no AMG wheels, kit etc which went for just under £16,000 in a private sale. Also a 30 odd thousand mile 1999 car with AMG wheels and kit for just under £17,000 and a 1998 23,000 special edition for £17,800 so good examples can fetch top prices.
I can fully understand why reputable specialists who properly prepare their cars and provide a comprehensive 12 month warranty charge a premium as most need money spending on them and can develop expensive issues which would result in a warranty claim plus they have had to go through the time, effort and expense of sourcing and preparing the car.
Excellent examples do come up from time to time at reasonable prices but you have to be prepared to spend a lot of time and effort finding them or be very lucky. You just can’t rely on what is said on adverts as in my experience more often than not, the reality does not reflect the description. I also think that sometimes honest sellers let their (excellent) cars go for less than they are worth due to pricing them based on other adverts they see.
I have certainly fallen foul of this in the past in that I valued my car based on those for sale of similar age, spec and mileage, not realising that mine which I believed to be in very good condition was comparatively fabulous compared to most others. I’ve also benefited from this once but usually narrowly missed out on such bargains.