If it isn’t, the seller must say why; like maybe the odometer is past its mechanical limits. Some older odometers only go to 99,999 miles and then start over at 0. Or, the odometer has been tampered with, broken or replaced.
If the seller tells you the mileage isn’t accurate, there’s not much chance of putting a good number to it; And there’s the unscrupulous seller who claims the reading is true, but it’s not so. What can you do?