Other options include:

• Local homeschooling groups / homeschool resource centers. Some homeschooling groups get on advanced topics.
• (For librarians) using lists like BACKMED-Exchange http://www.mlanet.org/page/backmed-exchange to see if other libraries have a use for that material
• Donating to a program like Better World Books https://www.betterworldbooks.com/go/donatebooks or other online book sellers/used book distributors
• (If the books are appropriate for the specific program's needs) donating to a books-to-prisoners program. A group I am involved with has a slightly dated list of programs at https://prisonbookprogram.org/resources/other-books-to-prisoners-programs/.
• Donating to a Little Free Library in the area https://littlefreelibrary.org/ourmap/
  

Thanks,

Erin