The scene is cut in a way that you don't know for sure they aren't paying him. They're probably pretty loyal, long-time customers given the way they all run right there after school, so he can take it on the honors system that they'll pay for what they're taking. (And he's probably able to keep track of it pretty well on his own given he knows their names and preferred candies by heart.)

As for the candy he throws aimlessly into the crowd, I'm sure he can afford to blow some pennies' worth of money on customer retention by literally throwing candy at them.