I would have thought that 'flat' is an acceptable (and reasonably universal) translation for דירה...
"Why sell this apartment?" Should be accepted. If not, add "are" to the word bank.
"Why DO you sell this apartment" isn't acceptable English.
'Why sell this apartment?' seems interrogative, as though the questioner is challenging the wisdom of the decision. 'Why do you sell this apartment?' feels a bit clunky, but is (I think) acceptable grammar. Perhaps a better translation would be 'Why are you selling this apartment?' (as you suggest.)