Taking photos at triathlon/sports events and selling photos online for profit?

The laws vary according to the country you are in. Basically, if it is a public place or you are in a public place and it is visible, you can use the first amendment protections for photography. The Electronic Freedoms Foundation has some good background information. The two big things are never give up your camera or film and always obey the nice policeman. Security guards can be intimidated with civil rights, the police know exactly where the line is.