Here, think of it as Sqegg's Comic Book. Just as with Schrödinger's Cat, where we do not know if the cat is dead or alive until we open the box, we do not know if you should keep or sell a book until we determine the fair market value. Until we do, it is equal parts sell and keep. In this unknown, yet known, state you can comfortably keep your book, or sell it, as determined by when the FMV is established.
If you want the cat to be alive, don't look in the box. If you want to keep the book, don't worry about its fair market value.