Sell used books
Selling used books has become a trade and a business in itself, no longer associated with selling books. Yes, there are many shops out there which solely sells used books and not brand new books wrapped in plastic sheets.
These shops usually get their used books from the ordinary man on the street who has books to sell. They will then of course price the book they bought it for slightly higher but definitely still within a very reasonable range. If you have books to sell, no harm bringing down a few to these shops to see how much they are worth.
All the white elephants in your mini library at home can definitely be put to much better use, such as converting it into cold hard cash. Having cash on hand is much better than having books that you have read and it being treated like gold. Contrary to what people think, knowledge from books is not gold. Cash is gold.