Almost every book somebody hopes is valuable is a later printing of a famous title, and almost every book that is valuable looks ordinary on a shelf.
What moves the number
Which printing, read off the copyright page rather than guessed.
The dust jacket — present, original, and whether the price corner is intact. For a modern hardback the jacket can be most of the value.
Signatures, and only what can be shown about them.
Condition, in the trade's own ladder, where good means worn. The ladder.
Why two of the same differ
One listing shows the copyright page held still for ten seconds. The other says "first edition" in the title. Buyers who know this category pay for the first and ignore the second.
How to find out rather than guess
Identify the printing and the publisher from the page itself. Then compare completed sales of the same printing with the same jacket situation.
A first with no jacket is not a cheap first — it is a different item, and should be compared against other jacketless copies.
What old books are usually worth
Less than people hope, because paper survives in quantity. A nineteenth-century book that everybody has is a common book. The money is in scarce printings, condition and association, not in age.
Questions people ask
Is an ex-library copy worthless?
No — it is a reading copy, and there is a market for those.
Does foxing destroy value?
It lowers it and is very common in Indian copies. Show it rather than hoping the photograph hides it.
What about a whole shelf?
Lots, by subject. Pull out anything with a jacket and a first printing and run it alone.