I took it to the local Volvo mechanic and he advised me to take it to a Speedometer repair shop. The speedometer repair shop gave me 2 options: $250 to fix if he has to remove and repair the speedometer unit; $100 to fix the odometer if I remove it myself and take it in. Apparently, it's some plastic gear that's broken. I'm gonna remove it myself; it's pretty easy since I've removed it to replace the burned out light bulbs.

I think it's better to repair the unit before you sell it because you'll have a hard time convincing a buyer you 9+ year old car has only 53k miles with a broken odometer.