Love that idea! We get so many books here. Both my mom and my mother-in-law were teachers, and they both discovered the USPS media mail rate. We get a box of kids books picked up from yard sales, library sales and such just about every week.

Adult books do better on the swap site, so I donate the books my kids have read already to their school. My son's teacher is in her 2nd year and can really use them.
In CA, we had a great used book store in town. There I would turn in all the used books for credit. I was one of their steadiest customers, but never spent more than a dollar or two on a visit. The owner didn't mind, we seriously provided over half of her kid's section! Here in PA, we don't have a used bookstore anywhere close.