Step 1: Rare book holders, know what you've got

From another article on protecting special library collections from theft, some tips that may also have application in being a rare book dealer- or a collector:

Steps to help prevent theft at your library:

1. Have your collection evaluated. If you don’t have a special collections librarian on staff, reach out to libraries in your region to find someone who specializes in the issue and have them look at your collection, says special collections librarian Bill Cook. Cook has evaluated collections at smaller libraries in his area and helped them identify what was most valuable.
   

2. Separate and protect. Once you’ve identified valuable items, put them in a protected place where they are locked and can be accessed only by a staff member with a key or ID. Keep good records of what materials patrons are accessing, and hold their library card or ID while they have materials in use.
   

3. Pay attention. “Short of an iron chain, the best way to prevent theft is [with] an attentive librarian,” says Travis McDade, curator of rare books at the University of Illinois at Urbana-Champaign’s law library. Have patrons use materials in a place where they can be observed, and don’t allow them to have bags or coats while accessing materials. Many thieves have used X-Acto knives to cut images, maps, and bookplates from rare books.

4. Be wary of technology, but use it to catalog. Most libraries can’t afford to spend thousands on security cameras and equipment, and McDade says it’s not necessarily better than having vigilant staff anyway. But if you can use technology to digitally archive your collection or keep better records, it can help.

5. Be a bit suspicious. Unfortunately, many library thefts are inside jobs, perpetrated by library staff members. The disappearance of thousands of volumes from the Girolamini Library in Naples, Italy, for instance, led to the library director, who was convicted and sentenced in 2013 to seven years in prison (which was later commuted to house arrest). Jerker Rydén, senior legal advisor of Sweden’s National Library, says as libraries increase security for patrons, a pattern of insider theft becomes more apparent. “[The thief is] usually the same person entrusted to safeguard something,” Rydén says.