There are things in my files that could net me more than a little money - maps, names of very important people I worked with or who were witnesses and much more- and I need the money. But it will never happen. If I reach the point and feel it better to destroy correspondence than let it fall into public domain then I will burn everything.
Whether the AOP Bureau work (despite betrayals by the authorities there) or exotic fauna I say "it's confidential" that is how it stays. It goes with me to the grave.