The used planes in career mode is not that spectacular now, as there is not much use in buying one if you have to repair it enough so that the costs are the same or more than the new plane.
Either way, the used catagory:
1.would better serve unique planes that need to be repaired before able to fly them, something like Gran Turismo’s used catagory where you could find cars that could not be found in the New catagory or Forza’s Barnfinds.
2.And maybe put the used catagory on a rotation.
Those two points makes the used catagory way more fun and usefull than what it is now.