"Thrifting" isn't just for broke college students anymore. According to a report from secondhand apparel site ThredUP, 67 percent of its most active shoppers are worth more than six figures. Even more surprising - 10 percent of its shoppers are millionaires, 21 percent have a net worth of $100,000 to $250,000, and 36 percent have a net worth of $250,000 to $1 million. Seventy-seven percent of all secondhand shoppers are working professionals, 71 percent own homes and 73 percent live in urban areas. These people are six times more likely to shop high-end boutiques than department stores, and two times more likely to shop online resale than online retail. The secondhand clothing industry is worth $18 billion, according to the report, which predicts the market will grow to $33 billion by 2021. Thrift shopping spans multiple generations - thirty-two percent of women over the age of 65 purchase used or re-sold clothing, along with 30 percent of millennials. Seniors opt for used clothing in favor of value and savings - versus millennials, who choose secondhand apparel as an effort to be more eco-conscious. Thirty percent of millennials ages 18-24 have shopped secondhand during the past year, and 21 percent say they plan to do so in the future. Ebay, Poshmark and LetGo are the most popular peer-to-peer marketplaces, while TJ Maxx, Marshall's and Neiman Marcus Last Call are the most frequented off-price retailers.