I am not really a shopper, but I enjoy a brief pass through a market or an antique district if I'm in the area. I pulled together the following info for a summer 2024 trip that included Milan. I used a lot of sources, including this forum, not necessarily current, so Google for confirmation. I cannot personally vouch for any of these places.
Markets:
Lombardy market listing for 2024: https://files.spazioweb.it/62/8b/628b5a9d-4a02-4c04-8bb3-0cd23ba0ee66.pdf
Mercatino Usato, Milano Centrale: Permanent indoor market with used goods of all types. Buys as well as accepting donations; quality generally high. Mon–Fri 1000-1300 and 1530-1930, Sat-Sun 1000-1900.
Mercato di Via San Marco, Piazza San Marco: Street market in Brera district, between Via Ancona and Via Montebello. Caters to wealthy residents of district. Organized by type of goods: food, household articles, clothing (cheap to cashmere), crafts, second hand and antique items. Mon and Thu 0800-1200. Metro Lanza, Moscova or Turati. From Lanza walking route is along Via Madonnina (beautiful architecture and design shops) and Via Brera (galleries). Fodor 2018.
Antiques market, Via Fiori Chiari, Via Madonnina and Via Formentini in Brera: 3rd Sun of month, 0900-1800. Jewelry, books, ceramics, clothing. Sometimes also artists. One source mentioned a 3rd-Sat market as well, but it's not in more recent listings.
Mercato di Artigianato e Collezionismo, Via Brera: 1st, 2nd, 3rd and 4th Tu and 1st Sun. 0900-1900. https://www.mercatiniecuriosita.com/milano/via-brera . Market running Aug 27 and Sep 3, 2024, 0800-1900.
Antiques market and collectors’ fair, Via Santa Maria Segreta: Sun.
Mercato di Antiquariato, Vintage, Collezionismo e Artigianato Artistico (formerly antique book market), Piazza Armando Diaz (Via Marconi arcades from Aregnario to Piazza Diaz): Sun all day. Not sure it's every month. Now seems a general antique/vintage/artisan market. https://www.mercatiniecuriosita.com/milano/piazza-a-diaz
Mercato di Antiquariato, Vintage, Collezionismo e Artigianato Artistico, Piazza Armando Diaz arcades from Piazza Duomo to Via Gonzaga and corner of Via Rastrelli): 1st, 2nd and 3rd Thu, but apparently not in August. See link above.
Laghetto Street Market, Via Festa del Perdono: Crafts and collectibles. Selected Wednesdays Feb-May. Along sidewalk near university. https://www.mercatiniecuriosita.com/milano/laghetto-street-market
Vintage and Artisan Market, Piazza San Nazaro in Brolo: Two Wednesdays per month (varying), excluding Jul-Aug. https://www.mercatiniecuriosita.com/milano/piazza-san-nazaro-in-brolo
Mercato Viale Papiniano: Neighborhood market Tue 0800-1500 (one source says 1300) and Sat 0800-1800 (also on Via B. Marcello); largest market. Both tourists and locals; crowded on Sat. Food, clothes (at least some secondhand), leather, jewelry, handmade art, cheap kitchenware, etc. Quality varies, but some bargains, especially on fashion (Fodor 2018 mentions apparel at Sant’Agostino end of market). Watch for pickpockets.
Pusterla Market for Artisans, Via Olona: 1st and 3rd Thursdays except Aug-Sep; 2nd and 3rd Thu instead in Dec. https://www.mercatiniecuriosita.com/milano/via-olona
Baracca Street Market, Piazzale Baracca: Crafts and collectibles. Selected Tuesdays Feb-Jun. https://www.mercatiniecuriosita.com/milano/
To be continued.