Other astronauts tried to take items like coins and stamps with them in order to profit from their sale. They were forced to give those up, and were in fact disciplined for violating NASA policy at the time, so there is some precedent here for NASA to stand on. I can see the tax dollar argument as well - it was public money, and Mitchell really shouldn't profit from selling it. Nonetheless, this is a PR disaster for NASA when they just can't afford one. They are going to look like creeps no matter how this suit turns out. Best thing to do would be to auction the camera, give Mitchell a "finder's fee" for his trouble, and split the proceeds between NASA and a charity of Mitchell's choice.