Some dealerships will buy cars that hit milestones like that to use as a display inside for the "longevity" of the brand. My local MB dealership has a 1979 VW Super Beetle Convertible that was traded in to them in 1992 with 64,000 miles on it. (has about 68,500 on it now) It is pristine and they keep it on display in the showroom as well as rent it out to special occasions around town. Pretty neat. Don't know if they'd buy a diesel MB with half a million miles on it for the same purpose though....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)