Short answer — the safest places to buy authentic luxury fashion online in/for Singapore are (grouped by type):

1. Official brand e‑boutiques and authorised boutiques (safest)

• Examples: Louis Vuitton, Hermès, Gucci, Chanel boutiques and their Singapore e‑shops or local boutiques. Buying direct from the brand’s Singapore site or authorised local boutique is the most reliable way to guarantee authenticity. (tatlerasia.com)

2. Leading global multi‑brand luxury retailers that ship to Singapore (new items; brand-supplied inventory)

• NET‑A‑PORTER (ships to SG; global luxury stock). (net-a-porter.com)
• Mytheresa (ships to Singapore; insured shipments). (mytheresa.com)
• SSENSE (inventory comes directly from brands/suppliers; product checks). (ssensesupport.zendesk.com)
• FARFETCH (marketplace of vetted boutiques; authenticity/customer promise and global shipping). (farfetch.com)

3. Well‑known regional platforms / department stores with curated luxury sections

• ZALORA — “Luxe” / designer edit in Singapore (works with authorised distributors). (zalora.sg)
• Lane Crawford (HK-based luxury department store that sells reputable designer stock and ships internationally). (lanecrawford.com)

4. Reputable pre‑owned / resale platforms with authentication services

• Vestiaire Collective (professional authentication before delivery; large community). (vestiairecollective.com)
• The Luxury Closet (global pre‑owned marketplace that offers authentication / lifetime authenticity claims; ships internationally). (sg.fashionnetwork.com)

5. Marketplace “official” channels (if you prefer local marketplace apps)

• Shopee Mall and LazMall: look for the Mall / Official Store badges — these mark brand-verified sellers and come with platform authenticity guarantees / buyer protections. (Useful for official brand stores or authorised distributors on those platforms.) (straitstimes.com)

Quick buying checklist (to reduce risk)

• Prefer brand e‑boutiques or authorised retailers (see above). (tatlerasia.com)
• On marketplaces, buy only from “Mall”, “Official”, or authorised/distributor stores (Shopee/Lazada) or from the platform’s authenticated listings (Vestiaire, Farfetch boutiques). (straitstimes.com)
• Check returns, authentication policy and insurance (high‑value items). Use platforms that inspect/authenticate items before shipping. (mytheresa.com)
• Pay with a credit card or PayPal for extra purchase protections and keep all invoices/serial numbers/packaging until you’re satisfied. (Standard consumer safety practice.)

If you want, I can:

• Make a short, prioritized list tailored to a category (handbags / shoes / ready‑to‑wear / pre‑owned) or
• Check a specific store or listing you’re looking at and flag any red flags.