Cancelation or not, there has always been money in buying and selling books, especially rare ones. It was the foundation of Jeff Bezos' empire after all.
The keys are the same as any other market.
Supply: 2 things will survive the apocalypse, cockroaches, and the millions of copies of The Da Vinci Code and Left Behind filling clearance bins in every used book store in America.
Pay attention to rare, hard to find, older books.
Demand: Either focus on the stuff everyone knows about - big name authors, popular topics - or something with limited demand, but the right buyer would pay a fortune for.
Condition: This one is the killer, especially for older books. People are rough on books, especially paperbacks. Old ones get mildew/mold and yellow, pages get folded or torn, they lose dust jackets.
My best tip? Auction houses, estate/yard sales are your friend. Book stores tend to know what they're dealing with, especially used or specialty book stores, and Amazon isn't going to haggle with you. People looking to offload grandpa's rare Civil War books, not so much.