Short answer — for guaranteed authentic new luxury buy from brand flagships or recognised authorised e‑tailers; for preloved luxury use established authenticated resale platforms. Recommended options (ships to Singapore or based in SG):

• Brand flagship e-shops / Singapore boutiques (best for authenticity and warranty): official sites of Chanel, Louis Vuitton, Gucci, Prada, Dior, Hermès and their Singapore boutiques. (Prefer brand sites or in-country boutiques for new items.)
• Global authorised multi‑brand retailers that reliably supply authentic new goods: Net‑A‑Porter, Mytheresa, Farfetch, MATCHESFASHION, SSENSE, 24S and Lane Crawford (HK). These source from brands/authorised boutiques and provide customer service for international deliveries. (brandradar.org)
• Major regional marketplace / department stores: Zalora (Luxe section) and established department stores that operate online in Singapore — useful when they list brand-authorised stock or have official brand stores on-platform. (brandradar.org)
• Trusted authenticated resale / pre‑owned marketplaces: Vestiaire Collective (large APAC user base, formal authentication and buyer protection) and other well‑known resellers with visible authentication policies. These are good for preloved pieces when you want documented authentication. (brandradar.org)
• Reputable local boutiques/resellers with in‑house authentication (use caution and check reviews): examples include LUX.R (Singapore-based preloved specialist) and other Singapore stores that publish their authentication process and physical inspection policies. Verify individual seller reputation before buying. (lux-r.com.sg)

Quick checklist to stay safe

• Prefer brand flagships or the authorised e‑tailers above for new items — they carry brand-supplied stock and warranties. (brandradar.org)
• For cross‑border purchases check whether checkout is DDP (delivery duty paid) so you won’t get unexpected GST/import bills on delivery. Big luxury e‑tailers usually explain shipping/fees. (brandradar.org)
• For preloved items, choose platforms that show a clear authentication workflow (digital checks + physical inspection) and offer buyer protection/returns. Keep all invoices, serial numbers and original packaging when available. (brandradar.org)
• Red flags: prices far below market, sellers who refuse returns or won’t provide provenance, poor reviews or no verifiable business address.

If you want, I can:

• Check which of the retailers above currently offer DDP shipping and returns to Singapore for a specific brand or item, or
• Pick the best single site for a particular designer (new vs pre‑owned) and verify their authentication/return policy.