Donate your old encyclopedias to a local school or library. This option depends on the age of the books and whether or not schools and libraries accept older sets of encyclopedias. Even if the school or library cannot use the books, they can sell them at school or library fundraisers.
Give them to a playhouse or theater. Your local theater may want to use them as part of a set for an upcoming play.
Contact your favorite charity and see if the charity has a thrift store that will take your donated encyclopedias. Your favorite charity might even donate the books to children in other countries that lack an adequate number of books from which to learn and read.
Sell your old encyclopedias at your next garage sale. If you have no plans for a garage sale anytime in the near future, ask a friend or family member if they would put your encyclopedias out for sale at their yard sale.