Selling second-hand books can be a lucrative side hustle if approached strategically. To begin, acquire books from various sources such as thrift stores, garage sales, library sales, estate sales, online marketplaces like eBay or Craigslist, book fairs, and even from acquaintances. Select a niche or genre you want to focus on, such as fiction, non-fiction, textbooks, rare books, or collectibles. Specializing in a particular area can help you cater to a specific audience.