Short answer — reputable options (new + pre-owned) that Singapore shoppers commonly use:

• Farfetch — global marketplace of vetted boutiques; guarantees authenticity and offers international delivery to Singapore. (farfetch.com)
• Mytheresa — curated, authorized luxury e‑tailer; explicitly states items are authentic and ships to Singapore (duties/tax info and delivery times shown on its Singapore site). (mytheresa.com)
• SSENSE — international authorized retailer (sells new items directly from brands), with product-inspection and authenticity assurances. (ssensesupport.zendesk.com)
• 24S (LVMH’s 24S) — LVMH-owned luxury site carrying designer stock from Paris (authentic, ships internationally) — verify returns policies on big-ticket buys. (buyandship.com.sg)
• Zalora (ZALORA Luxury) — regional e‑commerce destination with a dedicated “Luxury” section; many listings are sold/fulfilled by Zalora or authorized distributors. (zalora.sg)

Trusted pre-owned / authenticated resale platforms (if you want second‑hand):

• Vestiaire Collective — large European resale marketplace with a dedicated authentication team and in‑house checks. (vestiairecollective.com)
• The RealReal — U.S.‑based authenticated resale platform that offers international shipping and professional authentication. (website.therealreal.com)
• HuntStreet (regional) — Southeast Asia resale marketplace with local Singapore presence and in‑house quality checks / authentication workflow. (huntstreet.sg)

Local multi‑label boutiques / department stores with official online shops:

• Club 21 (Singapore) — long‑standing multi‑label luxury retailer with an online store serving Singapore; sources directly from brands. (club21.com