Isnt 팔지 마십시오 and 팔지 마세요 the same thing just different levels of politness?
why is it incorrect to say "안 파세요?"
its grammatically corrent, but it sounds weird and unnatural for koreans
"아기의 책을 안 파세요? " can translate (consider context) "I wounder you sell the baby's book." or "Do you not sell the baby's book?"---- So, It is wrong using in Korean.
why is 아기의 책을 안 파세요 wrong?
Because that's asking if they'll sell it.
May I ask whether "아기의 책을 판매하지 마세요" sounds awkward?
What's the difference between 사지 & 팔지?
They both mark correct...
사지 마세요 means do not buy. 팔지 마세요 means do not sell. It is marked as correct as the AI is treating it as a spelling error instead of a grammatical error.
Is "~팔지 마" unacceptable?
Probably coz they want us to be polite
It's fine. Flag it as an alternate translation
What is the difference between using 팔아 and 팔지 in this sentence? How does it (if so) change the meaning?
팔아 마세요 is not a word, basically. "~지 마" is the way to negate a verb or adjective.
Duolingo accepts this version as correct: 아기 책을 사지 마세요. Worth noting because, I think, it would be far more common in spoken Korean, and it's easier on the tongue. Please correct me, though, if I'm wrong on the first point.