An Indefinite Noun Can Become Definite by Direct Address

The Vocative and Definiteness

Now consider a common noun that was indefinite before the call:

وَلَدٌ - a boy, not identified.
يَا وَلَدُ - O boy! The speaker is addressing a particular boy.

In this taught case, direct address makes the intended person definite. The same applies to يَا رَجُلُ and يَا تَاجِرُ.