Hard to say. It's possible that if the crank was loose and the previous owner kept riding that they messed up the crank arm, maybe that's why he was selling.
If it's just loose, then tightening up will fix it.
If the crank bolt needs a hex driver, chances are it's 8mm. This is the most likely. Park Tool suggest a 10mm may be required, but I think this quite rare.
Redline lists 2 Monocog models in this years line-up, one uses a square taper BB, one uses ISIS.