Have you checked the very lively used CD market on Amazon and Discogs lately?
There are thousands of classical CDs commanding considerable prices. It does depend on composer, composition, performers, label, but a lot of repertoire, especially the more out-of-the-way stuff, is highly desirable.
Now, yes, selling them individually requires lots of work, if that's what you really want to do. My 1300 or so are staying put.