Did not accept: "Do they need to sell this? What is the sense of muszą? Need to, Have to, both carry the same sense of obligation.
Well, "need" is "potrzebować", but in this context it really would be rather translated as "muszą". Added.
What is the literal translation of muszą? I like to know a literal translation of a phrase or word which helps me understand and remember why it translates as such.
And a valid English question in this case is "Must they sell this?" (which is accepted).
That is possible, but quite unusual. "Do they have to..." would be very much more likely in normal speaking and writing. A past tense form of your suggestion exists, and is much more likely: "Have they sold this?"
Yes. Guess I agree with that, having given it some more thought....
EDIT: However, I have just found some examples of "Have I to...."; it's deemed quite old-fashioned though.
https://forum.wordreference.com/threads/have-i-to-or-do-i-have-to.351248/ . I can't find any immediate references to "have they to..." but I wouldn't be surprised if Jane Austen or somebody of the same era might have used it :-). Best avoided today, though!
Would "Do they have to sell it?" not be right also? Could 'to' mean it rather than this?