If someone says Eid Mubarak to you, it is polite to respond by saying ‘Khair Mubarak’, which wishes goodwill on the person ...
If someone says "Eid Mubarak" to you, a polite response would be "Khair Mubarak, " which wishes goodness and blessings upon the person who greeted you. You could also say "JazakAllah Khair," which ...