From what I gather, it can be quite thorny. From what I understand, even if the subject of the photograph is not recognizeable to the public, if someone recognizes themselves in an image from the place, clothing etc., it's dicey if there is a monetary exchange.. As I learned the hard way, it's always better to ask permission. I had spied an interesting piece of sculpture out in the open in a public space and took a picture of it. Ordinarily I would have just moved on but for some reason I sought out the sculptor and mentioned that I had taken a picture of several of his pieces and he might like a copy if any turned out well. He went ballistic. While he was OK with most of them, there was one piece he didn't want photographed. He insisted that he be shown the pictures that had been taken and wanted any that had been taken of that one piece deleted in front of him. His demeanor was such that I obliged..it seemed as though he would try to grab my camera and maybe get rough if I didn't.