This morning's NY Times article Selling Lesson Plans online Raises Cash and Questions, kind-a-sorta presents both sides.
Obviously, I think teachers buying and selling each others' products is a great way for us to help each other out. It isn't like teachers are paid nearly what they are worth. I am happy to pay a fellow teacher for a high quality, classroom-tested product, and I don't feel that I am entitled to her work for free just because we are in the same profession.
Personally, I'd rather have my child's teacher selling what she has developed while working in the classroom (on my tax dollar) than coming to work tired because she had to take a second job to pay the mortgage.