When you say "Les librairies vendent des livres" you don’t know precisely which books they sell. That’s why you must use the indefinite article des.

Have a look at those examples:

Les librairies vendent actuellement le dernier livre de Stephen King.

Les librairies vendent des bandes-dessinées d’Hergé.

Les librairies vendent le roman de André Dupont.

Do you feel the difference ?

• In the first sentence, you know exactly which book they sell, the last one of Stephen King.
• In the second sentence, you know they sell Hergé’s cartoons but that’s all. Not that precise.
• In the last one, it’s not as easy. Indeed, if you haven’t heard about André Dupont before you might not understand why I use le. But if you know that this man has written only one novel (or if you are a native and you will guess so ;), you will know which precise book I’m talking about.

That’s why the correct translation for "Les librairies vendent des livres" is actually "Librairies sell books".

NB: here André Dupont is a fictional character ^.^