You = "Profession + 님" is a polite way of addressing your interlocutor
손 = guest, visitor
손님 = you (only when the interlocutor is a guest or customer)
Addressing a total stranger is a bit trickier as it depends on gender, approx age group etc. of the 'stranger'. You can get round this by not using any form of addressing at all; but by using the appropriate verb ending to reflect the level of courtesy/respect you have toward them.