Third-party marketplaces concentrate most of the world's online shopping, and the model keeps expanding: social platforms now run marketplaces, US retailers are opening their marketplaces to international shoppers, and Latin America has become the most contested battleground. This FAQ covers how marketplace power is distributed, where the growth is, and what sellers should do about it in 2026.

What is an ecommerce marketplace?

An ecommerce marketplace is a platform where third-party sellers list and sell products alongside or instead of the platform's own inventory, with the operator providing traffic, payments, and often fulfillment and advertising. Amazon, Walmart Marketplace, Mercado Libre, and TikTok Shop are leading examples. The model dominates online retail because it concentrates selection and demand: In 2026 the top 18 ecommerce retailers will drive 74.1% of total retail ecommerce sales in the US, per an EMARKETER forecast. For brands, marketplaces are simultaneously a sales channel, an advertising platform, and a competitor, which makes marketplace strategy a board-level question rather than a channel detail.

How dominant is Amazon among ecommerce marketplaces?

Amazon remains the gravitational center of US ecommerce, holding 39.7% of all US ecommerce sales in 2026, with no other retailer close, per EMARKETER. That share underpins a second business: In 2026, Amazon accounts for more than 79.0% of all US retail media digital ad spending, since advertisers follow transaction volume. The concentration shapes seller economics across the industry. Brands typically need Amazon for reach, but the platform's fees, advertising costs, and control of customer data push many to diversify across Walmart Marketplace, TikTok Shop, and category-specific platforms. This suggests marketplace strategy in 2026 is a portfolio decision: Amazon as the anchor, with growth channels layered around it.

How are marketplaces expanding across borders?

Cross-border expansion is the marketplace battleground of 2026, with Mexico as the test case. Mercado Libre announced a $4.6 billion investment in Mexico for 2026, a 35.3% increase over the prior year and its largest annual commitment to the country. One day later, Walmart opened Walmart.com to international customers with Mexico as the first market, offering hundreds of thousands of US products with duties calculated at checkout. The stakes: EMARKETER expects Mercado Libre to generate $17.81 billion in Mexican ecommerce sales in 2026, 25.5% of the country's retail ecommerce, with Amazon at $13.84 billion and 19.8%.

“When Mercado Libre committed $2.5 billion to Mexico in 2024, its goal was partly to stay ahead of Amazon and Asian platforms,” said Matteo Ceurvels, EMARKETER principal analyst for Latin America and Spain. “The 2026 commitment nearly doubles that figure, and its growing share of the market suggests the strategy is working.”

How are social platforms becoming marketplaces?

Social commerce has produced the fastest-growing marketplace in US retail. EMARKETER expects TikTok Shop to generate $23.41 billion in US sales in 2026, making it the fastest-growing retailer EMARKETER tracks for the second consecutive year, with an ecommerce business larger than Target, Kroger, Shein, and Best Buy, per a June 2026 EMARKETER forecast. TikTok Shop's discovery-led model differs from search-led marketplaces: algorithmic content and creator affiliates drive purchases rather than shopper queries. Its success has pulled even multibrand retailers like Ulta and Sally Beauty onto the platform. This indicates the marketplace model is splitting into two architectures, search-based (Amazon, Walmart) and discovery-based (TikTok Shop), each requiring different seller capabilities.

What competitive pressures are reshaping marketplaces?

Marketplace competition is intensifying on two fronts:

• Low-cost cross-border players. Shein, Temu, and Shopee are pushing into Latin America and other regions, intensifying pricing pressure across categories.
• Regulatory scrutiny of platform tactics. Chinese authorities are scrutinizing sales tactics at Alibaba, Pinduoduo, JD.com, ByteDance, and Xiaohongshu amid weak domestic demand.
• Logistics arms races. Mercado Libre's investment plan spans technology, same-day delivery now available in 37 Mexican cities, financial services, and 8,500 new hires, showing how marketplace leaders convert capital into fulfillment moats.

Marketplace operators that cannot match logistics and pricing investment risk losing sellers and shoppers simultaneously.

How should brands and sellers approach marketplaces in 2026?

Run marketplaces as a diversified portfolio with channel-specific execution. Priorities grounded in EMARKETER analysis:

• Anchor on Amazon, grow elsewhere. Amazon's ecommerce share makes it unavoidable, but TikTok Shop's growth and Walmart's expansion reward early movers on secondary platforms.
• Prepare for cross-border selling. Brands on Walmart.com should verify international shipping eligibility and prepare Spanish-language listings as the marketplace opens to Mexico.
• Match capabilities to architecture. Search-led marketplaces reward listing optimization and retail media; discovery-led marketplaces reward creator networks and content velocity.
• Expect faster fulfillment standards. Sellers on Mercado Libre should plan for faster fulfillment timelines as logistics investments roll out, a bar other marketplaces will follow.
• Budget for marketplace advertising. With ad spending concentrated where transactions happen, organic visibility alone rarely sustains marketplace growth.

We prepared this article with the assistance of generative AI tools and stand behind its accuracy, quality, and originality.

EMARKETER forecast data was current at publication and may have changed. EMARKETER clients have access to up-to-date forecast data. To explore EMARKETER solutions, click here.