I think that this is completley legal. Teachers create their own lesson plans and sell them on a legitaamite website to other teachers. The ethics of the situation are different. An arguement could be that teachers are paid to teach students and creating those lesson plans are part of their job, so buying lesson plans from other teachers could be seen as not really doing what they were hired to do. I do understand why teachers would sell and buy them though. Teachers do not make that much money so they can make more from selling the lesson plans. The teachers that buy the lesson plans could be busy or on a tight schedule and would be unable to make there own. Or they just want to get ideas from other teachers that will improve the lesson for the students.