If you are going to sell it, you should not do anything to it. Don't paint it, or take it apart, or anything except maybe oiling it.
Whoever buys it will have their own technique for bringing it back to life.
Many times a well meaning person who is not familiar with restoration/repair techniques of trains of a certain era will do something that they think is a good idea, but actually damages the set - sometimes beyond repair.
In other words, let the buyer work on it.
It's just my opinion, but my opinions come directly from my experience.