The following comes from my article in NEA TODAY. (September 2001)
I ask my fourth graders to bring in books that they've already read. Then we set up displays for those who brought books to sell. Booksellers also get a chance to circulate and buy books from others. Since there are at least five book exchange days in a school year, the students get to resell many of the used books that they bought. I've found that it is good to set a price limit on what a used book may sell for. Thirty minutes is plenty of time for a book exchange period. Parents are especially happy that their children are getting to read many books at little or no cost.
Ed Chappie (firstname.lastname@example.org)
Hope you can use this. The students loved it because they got to read books that they chose.