If I got it right, French must always have some article before a noun. On the other hand, English does not use articles when the speaker means all items of sth in general. So, sometimes you have to add one when translating from English to French:
I love books A and B = J'aime les livres A and B
I love books (books in general) = J'aime les livres.
Libraries are always full = Les librairies sont toujours pleines
That is OK. However, I found this sentence:
Les librairies vendent des livres = Libraries (in general) sell books (in general)
Why the indefinite article is used instead of the definite one? I'd understand using "des" with the meaning of "some" as below:
Ce magasin vend des livres = This store sells some books.
However, I do not see that usage in the previously mentioned sentence. It seems to mean "Libraries sell some books", which does not make sense to me.