It is your right to do so. This applies to any photos you take of anyone in public. As long as you are not selling them for commercial purposes (e.g. used for advertising a product or service in a brochure, magazine ad, television commercial, etc.), you are free to sell such images.
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).