Is there anything due upon me if someone, out of courtesy, addresses me as "Ya Sayyid al-Nas" (O Master of the People)?

There is no harm in addressing a person as the master of people (sayyid an-nas) or the master of the folk (sayyid al-qawm), as long as the intention is not that he is the master of all humankind, for that title is exclusive to the Prophet, peace and blessings be upon him. The Prophet's statement to the Companions, "Stand up for your master," referred to Sa'd ibn Mu'adh, and his statement, "Al-Hasan and Al-Husayn are the masters of the youth of Paradise."