That would be Capitalism. That's EEEEEVIL!...
It’s a school. there’s no particular reason to allow students to run a candy store during school hours on school property.
If the kid wants to sell candy door to door, or on the street corner, go right ahead. There’s no particular reason why just because capitalism is good, it means we have to allow it to operate everywhere. Sex is good, but I don’t expect people to be doing it at school.