I sell books on Amazon and have done so for many years.
Several times, I have sold early/old editions to the authors, but resisted the urge to squee!
I wanted to, but some kind of shyness creeps in and I stop, with the excuse to myself that I don't want to be considered unprofessional. even though it probably isn't unprofessional.
A few years ago a very famous Broadway producer purchased an old book on a rather controversial topic, and I wondered if this book was somehow research for a future show. I still check out this producer's plays to see if that topic has been part of any recent plays, but so far, nothing.