This happened to me with the SL500 and we eventually got to the bottom of it.
If you offer the car to a dealer as a PX and they do an hpi check they can put in the milage in the search to get a value on the car based on milage.
Now if you have a car that has done 27,000 miles on wednesday when one dealer did a search... then on the friday another one does a search but puts in 57,000 miles for some weired reason,then the Hpi updates the milage to the new figure.
So when your private buyer does his Hpi check he gets the discrepency from what you are saying and what the dealership stated was the milage.
I had to threaten Hpi with legal proceedings if they did not correct the mistake and at first they wanted all my service records and old MOTs.
I told them that i would not do this but if they called Mercedes Exeter they would confirm that the milage cannot be tampered on an SL500 ,but also if they checked the last few Hpi checks they would see that 3 showed 27,000 in a few days and then the milage shot to 57000 in less than a week.this was not possible and that the onus was on them to correct false information because if my buyer walked i would sue!
They sorted it within 48 hours and even phoned me back to confirm it had been corrected.
