Short answer — the biggest Malaysian apps/marketplaces where people commonly sell Cartier watches or affordable “Cartier‑style” / Tank‑style watches are:

• Shopee (Malaysia). (shopee.com.my)
• Lazada (Malaysia). (lazada.com.my)
• Carousell (local second‑hand / classifieds — many Cartier and Cartier‑look listings show up). (carousell.com.my)
• Mudah.my (Malaysian classifieds; pre‑owned Cartier listings are frequently posted). (mudah.my)
• PGMall (local marketplace with watch sellers). (pgmall.my)
• Chrono24 (international pre‑owned marketplace that lists Cartier watches available to buyers in/for Malaysia). (chrono24.my)

A few quick tips and legal/quality notes

• Taylor Swift’s pieces are genuine Cartier (she’s been photographed wearing Panthère / Santos models). If you want the exact look, the official Cartier stores or authorised dealers in Malaysia sell authentic models. (businessinsider.com)
• These apps include a mix of: official retailers, reputable second‑hand dealers, private sellers — and also unbranded/replica items. Exercise caution: look for seller ratings, authenticity paperwork, and preferrably buy from authorised dealers or trusted second‑hand shops. Cortina/other authorised dealers list Cartier lines in Malaysia. (cortinawatch.com)
• If you want an affordable, legal "Cartier‑style" alternative rather than a counterfeit, consider rectangular/“Tank‑style” models from mainstream brands (examples sold in Malaysia: Tissot Heritage Porto, Hamilton Boulton, Seiko SUP880, etc.). These give a similar aesthetic without legal/quality risks. (tissotwatches.com)

If you want, I can:

• Look for current listings of Tank/Santos/Panthère‑style watches on any one of those apps and show a few options (with prices and seller details), or
• Find authorised Cartier retailers in Malaysia and their app/online storefronts so you can compare authentic vs affordable alternatives.