"How much are you selling them for?"
Translation:Combien les vendez-vous ?
"les" is a plural direct object so in a question you have to place it before the verb, and the verb has to be placed before the subject. If there is a question word you should start with it. In summary here is the right order: (question word) ; (direct object) ; verb ; subject
Depending on the context "you" can be translated as: - "tu" (informal you, always refers to a single person) - "vous" (formal you that refers to a single person or many people)
I wrote "Pour combien les vendez-vous?". Correct and accepted. No problem.
it is "Combien les vendez-vous?" that worries me. (the DUO's translation).
I am not sure if you can say in English "How much are you selling them"? (without the "for"). But I am.pretty sure that you can't say in French "Combien les vendez-vous?" (without the "pour")
Does any one really know?
In this case, leur would indicate that you are selling someone/a person! :P Leur is generally reserved for possession: leur chat = their cat. Les can be used to represent people (them) in a case there they/them is the direct object, as in "Je les vois" - I see them. Je le les donne - I give it to them.
I think your answer is not totally correct
1 - «les» : pural direct object Je donne un livre / je le donne / i give it.
je donne plusieurs livres / je les donne / I give them.
2 - «leur» (indirect object) . Je leur donne un livre / I give a book to them / je le leur donne.
I give some books to them / je leur donne quelques livres / je les leur donne.