"Do they sell books?"
Translation:Eles vendem livros?
No guys, you can't do that (I mean, you can, but...). If you say "Vendem livros?" it can mean both "Eles/Elas vendem livros?" OR "Vocês vendem livros?", so unless the context is already given (which in Duolingo, it never is) to the person you are talking with, you do not omit the pronoun. Also, people tend to think that just because in spanish they omit a lot their pronouns we do the same in portuguese. Well, maybe in Portugal they do, but in Brazil we use the pronouns a lot. We do not always omit it.
In this case, it's the same as English.
Do they sell books? -- Generic. No book specified. Wants to know if they sell books in that store, for example.
Do they sell the books? -- This means speaker and listener know which books are being mentioned. If they don't, this will be immediately followed by "What/Which books???"
Some other cases might be tricky, but usually, when it's not very clear it means 1) it's an expression/it's the way it is, there's no proper grammar explanation; OR 2) it doesn't really matter xD
But it mostly works on a case by case basis, so... =\