What Are 3rd Party Sellers? Amazon and Walmart Marketplace Explained

Third-party sellers are individuals or companies that sell products on Amazon and Walmart without being the platform itself. They can be small businesses, large conglomerates, private label brands, or individual entrepreneurs — anyone who uses Amazon or Walmart's marketplace to sell directly to customers. According to Amazon's investor reports, third-party sellers now account for over 60% of all units sold on the platform — making them the backbone of the Amazon marketplace.

How Third-Party Selling Works

Amazon and Walmart allow third-party sellers to list products on their websites, giving access to hundreds of millions of active shoppers. To list a product on Amazon, sellers provide product information, upload images, set a price, and choose a fulfillment method. The same core process applies to Walmart, though with different requirements and approval processes.

The key advantage: instead of building your own eCommerce website and driving your own traffic, you tap into the existing customer base of the world's largest online marketplaces. This is why starting to sell on Amazon is often a faster path to revenue than building an independent store from scratch.

Amazon vs. Walmart: Key Differences for 3rd Party Sellers

Market Size and Competition

Amazon commands approximately 37% of all US eCommerce sales, with over 200 million Prime members. Within the platform, 65% of US consumers shop on Amazon regularly versus 37% on Walmart.com. However, this also means Amazon is significantly more competitive — thousands of sellers may be competing for the same keyword.

Walmart.com, while smaller, is growing faster. With significantly less competition, third-party sellers often find it easier to achieve visibility on Walmart than on Amazon for equivalent products. This is why Walmart marketplace management has become increasingly valuable alongside Amazon.

The Buy Box

The Buy Box is the "Add to Cart" button on Amazon product pages. When multiple sellers offer the same product, Amazon's algorithm selects which seller wins the Buy Box — and that seller captures the vast majority of sales. Winning the Buy Box requires competitive pricing, strong seller metrics, and typically FBA fulfillment. Walmart has an equivalent system that operates on similar principles.

Fulfillment Options

• Amazon FBA: Ship inventory to Amazon's fulfillment centers; Amazon handles storage, shipping, and customer service. Prime badge included.
• Amazon FBM: You fulfill orders yourself from your own warehouse.
• Walmart WFS: Walmart's equivalent of FBA — send inventory to Walmart's warehouses, get the 2-Day delivery badge.
• Walmart DSV: Drop-ship from your own warehouse directly to Walmart customers.

For most serious sellers, FBA and WFS are the preferred models because the fulfillment badges dramatically increase conversion rates.

How to Become a 3rd Party Seller on Amazon

Go to sell.amazon.com, create a Professional Seller account, and provide your business information, tax ID, bank account, and identity verification. The Professional plan costs $39.99/month but is essential for scaling — it unlocks PPC advertising access and removes the 40-item selling cap. See our complete Amazon selling guide for the full process.

How to Become a 3rd Party Seller on Walmart

Walmart's application process is more selective than Amazon's. You'll need:

• US Business Tax ID (SSN not accepted)
• W9 or W8 and EIN Verification Letter from the Department of Treasury
• US business address for physical operations
• Planned catalog integration method (bulk upload, API, or solution provider)
• Primary product categories and catalog size details with verified UPC information

The approval process typically takes 2–4 weeks. Working with a Walmart agency can help streamline the application and ensure you meet all requirements on the first submission.

Both Amazon and Walmart offer distinct advantages. The smartest brands in 2026 don't choose between them — they sell on both, using each platform's strengths to build a diversified, resilient eCommerce business.

Should You Sell on Amazon, Walmart, or Both?

The answer for most established brands is both. Amazon provides unmatched traffic volume and the Prime ecosystem. Walmart provides a Blue Ocean opportunity with less competition and comparable fulfillment infrastructure through WFS. Our Amazon + Walmart management service is specifically designed for brands ready to maximize both platforms simultaneously.

Ready to start or scale your third-party selling business? Contact Prolific Zone for a free consultation — we'll assess your products and build a strategy for both platforms.