Vintage Synth Repair

There's a Yamaha AN1X keyboard on eBay which the seller claims has one key that always plays at max velocity regardless of how hard or soft it is pressed.  Does anyone have any thoughts about what could be causing this and what could fix it?
  As I understand, velocity on a keyboard is measured by having two switches.  One activates at the beginning of the keypress and the other activates at the completion of the keypress.  The velocity value is based on the time between the two switch activations.  If the bottom switch were broken to be never activated, it would appear that the player is taking infinite time to depress the note, and, I would guess, the velocity would be zero for this key.  It would never sound.
  If the bottom switch were always activated, pressing the key would appear to have zero time between the top switch activating and the bottom switch activating, giving instant keypress and max velocity.

  The Yamaha AN1X is a MIDI based late 1990s Analog Synth Modeller.  The opening bid is $200.  Does this seem like a reasonable buy according to the collective wisdom here?

Thank you.