Most commonly in physical stores, one often finds that, in a multi book series of books, volumes are missing or not available, good shop managers may be trying hard to avoid that situation, but I think it is a pretty common thing. This problem can only be worse with comics, graphic novels, most of which come in serial volumes.

Another is the available of cheaper international paperback editions online.

For translated books, I do not find bookshops striving to carry as many different translations of the same work as possible, most carry only one, when some works like Homer and the Russians Greats are published in very many translations.