Intangibles

     "Probably we will take him on an inspection tour of the ship before it takes off and give him, absolutely free, a picture of himself standing in front of it, with the pilot's own signature signed across the bottom by some female clerk."
    "Chiseling from kids. Bah!"
     "Not at all," answered Montgomery, in hurt tones. "Intangibles are the most honest merchandise anyone can sell. They are always worth whatever you are willing to pay for them and they never wear out. You can take them to your grave untarnished."

Robert Heinlein
The Man Who Sold the Moon (1949)