It depends on what you’re collecting but for pre-ISBN books, AbeBooks is the central marketplace for rare books so that’s one place to look up titles but most book collectors specialize, often in a very specific niches like certain authors or topics, usually restricted to a single era.

The hard part is tracking editions and verifying that you have a genuine first edition. Collectors depend on bibliographies which are themselves rare books that describe all the publications of a specific publisher or author and how to identify editions. Often there are marks but for valuable books it usually requires knowing the exact typos and changes between editions to authenticate them. There are a lot of counterfeits, both modern and contemporary to the original printing.

The other problem is that the vast majority of books are worthless unless they’re in very good condition. Not “good condition for their age” but damn near pristine, even if it’s hundreds of years old. If it’s falling apart, it’s almost certainly worthless, unless it’s an honest to god manuscript.

It’s mostly dependent on luck. It’s really hard to make a living with book collecting unless you’re running an auction house. Most companies handling estate sales are smart enough to check AbeBooks so the lucky life changing finds are getting rarer and rarer.