No one said you did. As Hussar said, it could be the internet... Or it could be a phone book. Some sort of information transmission system that goes beyond putting your sign up above the door.
Except that, contrary to what Hussar said, people can and do advertise without newspapers, telephones and the internet.
Especially in DnD where, with magic, you can have a 101 different ways of getting word out there ranging from hiring a Bard to spread the word around town all the way to summoning an Air Elemental to write your message in the Sky.