It depends on the custom of that area of the city. And it depends on your courage in the face of police harassment.
There is a Federal Appeals Court ruling which states that artistic expression is protected by the 1st Amendment and the artist needs no permit to sell their artwork on public lands
This ruling was appealed by the City of New York. The Supremes refused to even grant them a Writ of Certiorari, thereby upholding the Lower Court decision.
This ruling is a huge weapon in the pocket of the artist who wishes to sell his own artwork on public lands.