Values aren't determined by what people think they should be, they are determined by what people are willing to pay for the goods/services in question. In this case, I too think $18K is way too much for a 15 year old W126 diesel sedan even if it has only 72K miles on it. However, if someone is willing to pay it, then that's fine. That's how the market goes. Good for the seller anyway.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver