there are spots around the city where you find informal flea markets set up on weekends. It may just be 2 or 3 folks selling various odds and ends. Usually in front of a closed public building like a school or library, so nobody in the building to call and complain to the cops. Not to mention your typical stoop sales or garage sales (in parts that have garages). Some of these stoop sales are every week at the same location (weather permitting), wih craigs list ads announcing them. Have not gone to one of those. With that said, if you have a TAX ID and sell books/literature, the police are kind of limited in how much they can force you to move if you are not impeding traffic. First Amendment and such.
As I've discussed here before, there is a guy in my area who sets up, sells comics, cards, stamps, coins, postcards, whatever. He's a retiree looking for some extra income. My chance to snag some comics at a decent price from his has passed as now he looks most stuff up on ebay. He claims he bids on lots on ebay to get merchandise.