Magic Collecting is a relatively recent phenomenon.
While there were a few collectors 100 years ago, there were so few that they were known to everyone: Saram Ellison was one. When he tried to eventually sell his collection through The Sphinx, there were no takers so he left it to the New York Public Library. Is it still there?
Chung Ling Soo posters which now bring thousands of dollars were, only 40 years ago, being used by Davenports of London to wrap parcels of magic being shipped to mail-order customers.
The field of magic collecting has exploded in the last few decades.
Major collections in the United States are held by George Daily, Jr. (magazines and Houdiniana), Tad Ware (illusions), Ken Klosterman (general), Bill King (20th century magic), and Ray Goulet (P&L, and Ray has a wonderful museum in Watertown, Mass.), William Kalush (books), among many others. (Feel free to edit this list and add more names and their specialties).
The largest collection in the United States belongs to David Copperfield, whose collection should rightly be called The Metropolitan Museum of Magic. It is a staggering collection encompassing every type of collectible in the field of magic, all in massive quantities.