Contact your local library. Many libraries around the country have an ongoing book sale where they sell books that have been donated to the library. Additionally, many libraries keep community bulletin boards on which you can place an announcement letting people know that you’d like to donate your used homeschool books.
Advertise on a homeschooling message board, including on social networking sites like Facebook. Many exist across the country. Their purpose is to provide support of all kinds for parents who homeschool their children, including providing parents with information about books and supplies.
Post flyers on church and community center bulletin boards. For those who home school their children and who want a religiously based curriculum, this offers you a chance to get materials for lessons as well as give books and other materials away.