Manga prices on which release you have and on whether the set is whole, far more than on the title.
What moves the number
Which edition — standard, deluxe, omnibus, collector's.
Publisher and language.
Completeness, stated by volume number rather than as "nearly complete".
Condition — spines first, then page edges and covers.
Dust jackets, where the edition had them.
Why two of the same differ
One listing shows the publication page and the spines with the numbers facing the camera. The other says "set of manga". Describing manga condition.
How to find out rather than guess
Compare completed sales of the same edition, complete runs against complete runs. A partial set compared against a complete one gives a number that will not happen.
If your run has gaps, check what the individual missing volumes sell for — frequently the gaps are worth more than the rest, and selling singles beats selling an incomplete set.
Figures and merchandise
Box condition, completeness and sunlight. The toys guide applies.
Questions people ask
Does an unofficial printing have value?
Little, and it must never be sold as official. Counterfeits policy.
Is yellowing a problem?
It lowers the price. Show the page edges rather than arguing about the word.
Should I split my set?
If volumes are missing, usually yes. Complete runs sell best whole.
More: pricing anime and manga, selling them.