You do not need permission from someone to take their picture if they are in public view. However if they in an area where they have a reasonable expectation of privacy, you are not allowed to invade that privacy by photographing them. The article covers privacy laws of all 50 states.
What Sells Best On Shutterstock?
Some types of photos that sell best are Portrait Photography, Food Photography, Landscapes (usually with people) and fine art photography (artsy).