Kavitha Cardoza:

Some have even become millionaires, including a kindergarten teacher from Florida, an elementary school teacher from California, and an English teacher from Louisiana.

These online marketplaces are becoming more and more popular. But there are also concerns. Some legal experts say, if a teacher creates educational materials, those materials legally belong to the school district.

Some educators worry about quality. And there are those who question what this means for the teaching profession, which traditionally has shared these materials for free.

Bob Farrace is with the National Association of Secondary School Principals. He worries this trend could discourage teachers from working together.