Used book sales
A few CPL branches have book sales each year and some street-fests may have used book sellers [Lisanote: the Near North Branch does this 2x a year. Look for signs inside!].¬† If you are a book sale junkie, Book Sale Finder is a useful Web site.
The Printers Row Book Fair, held in June each year, also has some used book sellers. I‚Äôve also seen some garage/sidewalk sales in the city, so keep an eye out for those too.
Also, thrift stores also often have book sections.