I'm not sure about Amazon but I do not believe that eBay's concern is jr high or high school kids getting a copy of the books and cheating. 1- the kids would have to have thier parent's credit card or Paypal 2- Doesn't a person have to be 18 to have a eBay account. 3- eBay sells sex toys, racist items and a million other items teens shouldn't get but they don't care so why should eBay care if the kids cheat? PLUS, just because a book is the teacher's edition does not mean that all the answers are there. Kids do have to write essays and stuff like that too that aren't in the book. I think that eBay gets some type of kick-down or money insentive to not let people sell any type of homeschool teacher/parent edition items at all.
Just my 2 cents!