I seldom buy used books. My book-buying sweet-spot is a remaindered book at a nice discount from the original suggested price. Still, from time to time I feel that I really need to possess a book that's been out of print so long that new copies are non-existent and the chance of a new edition seems close to nil.
Examples are the two editions of Frank Wootton's "How to Draw Cars" from the early 1950s. I never bought them when I was young, finally indulging myself last year with used copies shipped from Australia and England.
This was fine because the Wootton books were illustrated in black-and-white. Old books about painting are another matter because paintings are in color and it's important for apprentice-painter me to view another artist's color usage as accurately as possible. The problem here is that color reproductions from 15-20 years ago and before had what I consider iffy quality.
This means that while I can locate on the Internet some pre-1990 books about obscure artists that interest me, I'm not willing to buy them because I'm unsure if the reproductions will be helpful.
Now that we're in the age of digital photography, it's possible to visit museums friendly to non-flash picture-taking and get needed details to store on one's computer -- the main problem being to get to the desired museums.
The Internet is rapidly becoming a useful source for viewing reproductions, but matters of copyright, the fame of the artist and policies of museums and galleries have kept it in mixed-bag status.
Despite the problems and limitation noted above, art students and fans are vastly better off than they would have been 150 years ago when just about the only source of information regarding distant paintings consisted of engravings.
As for old art books, I'll probably buy one from time to time. But only when the text is important or the art was monochrome in the original.
[Cross-posted at 2Blowhards.]