Isnt 팔지 마십시오 and 팔지 마세요 the same thing just different levels of politness?
why is it incorrect to say "안 파세요?"
May I ask whether "아기의 책을 판매하지 마세요" sounds awkward?
why is 아기의 책을 안 파세요 wrong?
To me your sentence sounds like "he doesn't sell the baby's book." Whereas the connotation of the sentence is more of a command: "don't sell the baby's book."
Is "~팔지 마" unacceptable?
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.
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.