Short answer: mainly on big marketplaces, a few fashion‑focused sites, and resale apps. The places Singaporeans commonly buy clothes online are:

• General marketplaces (most popular): Shopee and Lazada — where many local sellers and brand stores list clothes. (statista.com)
• Fashion specialists / multi‑brand retailers: Zalora (largest dedicated fashion site in S’pore) and local brands like Love, Bonito. These focus on apparel, labels and faster local returns. (sem.jupiterseotool.com)
• Second‑hand / classifieds: Carousell is widely used for pre‑owned and budget buys. (mediaonemarketing.com.sg)
• Cross‑border / bargain platforms: AliExpress, Taobao (and services/aggregators that handle shipping), plus global retailers that ship to Singapore (ASOS, H&M, Uniqlo, Zara, Amazon). (These are common for niche items or lower prices.) (statista.com)

Quick tips:

• For local delivery and easier returns, look for Singapore‑based sellers or the site’s Singapore storefront.
• Check size charts (brands vary) and seller ratings on marketplace listings.
• If you want sustainable/unique items, browse Carousell or small local boutiques/Instagram shops in addition to the big platforms.

If you want, I can: show the top 10 sites with traffic numbers, or list links and return policies for a few retailers. Which would help most?