How Sellers.json Works

Sellers.json files are hosted by SSPs, ad exchanges, and ad tech platforms at a standard location: domain.com/sellers.json. Any buyer, verification service, or industry participant can access these files to see who sells inventory through each platform. The standard provides structured, machine-readable information about seller identities and relationships.

When buyers evaluate inventory in RTB auctions, they can check sellers.json to verify seller identity, confirm authorized relationships, identify resellers versus direct sellers, and understand supply chain composition. This transparency helps buyers make informed decisions about inventory quality and source.

File Structure

Sellers.json follows a standardized JSON format specified by the IAB Tech Lab. The file contains a version number indicating the specification version, a contact email for questions, and an array of seller objects with detailed information about each seller.

Example Sellers.json Structure 
{
  "version": "1.0",
  "contact_email": "[email protected]",
  "sellers": [
    {
      "seller_id": "pub12345",
      "name": "Example Publisher Inc",
      "domain": "publisher.com",
      "seller_type": "PUBLISHER",
      "is_confidential": 0
    },
    {
      "seller_id": "reseller789",
      "name": "Media Reseller LLC",
      "domain": "reseller.com",
      "seller_type": "INTERMEDIARY",
      "is_confidential": 0
    },
    {
      "seller_id": "pub98765",
      "seller_type": "BOTH",
      "is_confidential": 1
    }
  ]
}

Required Fields

seller_id uniquely identifies the seller within the platform's system. This ID matches the publisher/seller ID in ads.txt entries. seller_type categorizes the seller as PUBLISHER (direct seller of their own inventory), INTERMEDIARY (reseller of others' inventory), or BOTH (selling both own and others' inventory). is_confidential indicates whether seller identity is public (0) or confidential (1).

Optional fields provide additional transparency. name gives the seller's business name. domain identifies the seller's website, enabling verification against ads.txt. Including these optional fields demonstrates commitment to transparency and helps buyers evaluate seller legitimacy.

Cross-Reference with Ads.txt

Sellers.json and ads.txt work together for complete verification. Check seller_id and domain in sellers.json, then verify those sellers are authorized in the publisher's ads.txt file. This cross-reference catches unauthorized reselling and validates supply chain legitimacy.

Why Sellers.json Matters

Supply Chain Transparency

Before sellers.json, buyers often couldn't identify who actually sold inventory they purchased. Resellers operated anonymously. Supply chains contained multiple undisclosed intermediaries. Unauthorized reselling was difficult to detect. Sellers.json makes the supply chain visible, enabling buyers to understand exactly who they're buying from.

This transparency benefits the entire ecosystem. Publishers see who resells their inventory and can verify authorization. Buyers understand supply paths and can optimize toward direct relationships. Platforms demonstrate commitment to accountability. Bad actors face exposure when operating without authorization.

Fraud Prevention

Sellers.json supports fraud prevention by exposing unauthorized reselling. When buyers check sellers.json against ads.txt, discrepancies reveal potential fraud. A seller listed in sellers.json but missing from a publisher's ads.txt indicates unauthorized reselling. Confidential sellers claiming to represent major publishers warrant investigation. Anonymous or fake seller identities become apparent.

While sellers.json doesn't directly prevent invalid traffic or bot fraud, it creates accountability that discourages fraud. Transparent supply chains make it harder for bad actors to operate. When combined with comprehensive fraud detection, sellers.json contributes to overall ecosystem quality.

Buyer Confidence

Buyers increasingly demand supply chain transparency. Sellers.json demonstrates platforms' commitment to accountability and quality. Platforms publishing comprehensive sellers.json files with complete information signal transparency and trustworthiness. This builds buyer confidence and competitive advantage.

Conversely, platforms not publishing sellers.json or listing many confidential sellers raise buyer concerns. What are they hiding? Why the opacity? Buyers may reduce spending or avoid platforms that resist transparency standards.

Sellers.json vs Ads.txt

Sellers.json and ads.txt serve complementary functions in supply chain transparency. Understanding how they work together is essential for effective verification.

Different Perspectives

Ads.txt is publisher-facing. Publishers host ads.txt files on their domains listing which platforms are authorized to sell their inventory. It answers the question: "Who can sell this publisher's inventory?" Publishers control ads.txt and use it to authorize specific sellers.

Sellers.json is platform-facing. SSPs and exchanges host sellers.json listing all sellers using their technology. It answers the question: "Who are these sellers?" Platforms control sellers.json and use it to identify their sellers publicly.

Verification Flow

Complete supply chain verification uses both standards together. Buyers see inventory in an auction from a specific seller ID on a specific platform. They check the platform's sellers.json to identify who that seller ID represents—getting name, domain, and seller type. Then they check the publisher's ads.txt to confirm that platform and seller ID are authorized to sell that publisher's inventory. If both checks pass, the supply chain is verified. If either fails, unauthorized reselling is occurring.

Complementary Roles

Ads.txt prevents unauthorized selling by letting publishers explicitly declare authorized sellers. Sellers.json prevents anonymous selling by requiring platforms to identify their sellers. Together they create accountability on both sides—publishers control who sells their inventory while platforms disclose who uses their technology. This dual-sided transparency makes unauthorized reselling visible and accountable.

Both Standards Required

Neither ads.txt nor sellers.json alone provides complete supply chain verification. Ads.txt without sellers.json enables anonymous selling. Sellers.json without ads.txt can't confirm authorization. Both standards working together create the transparency needed for supply chain accountability.

Implementing Sellers.json

For Ad Tech Platforms

Platforms implementing sellers.json should follow IAB Tech Lab specifications precisely. Host the file at https://domain.com/sellers.json as a publicly accessible JSON file. Include all sellers using your platform—publishers, resellers, and intermediaries. Provide complete information when possible, including names and domains rather than marking everything confidential.

Update sellers.json regularly as sellers join or leave the platform. Many platforms update daily or weekly. Include contact information so buyers can ask questions. Consider implementing sellers.json alongside ads.cert for enhanced verification. Document your seller onboarding process to ensure only legitimate sellers get listed.

For Buyers

Buyers should incorporate sellers.json checking into supply chain verification workflows. Download and parse sellers.json files from platforms in your supply path. Cross-reference seller IDs with ads.txt files. Flag discrepancies for investigation—sellers listed in sellers.json but not in ads.txt warrant scrutiny, as do excessive confidential sellers or missing seller information.

Build or license technology automating sellers.json verification. Manual checking doesn't scale to thousands of daily transactions. Automated verification can check every impression or sample representative inventory. Many DSPs and verification vendors provide built-in sellers.json checking.

Best Practices

Platforms should maximize transparency by providing complete seller information, minimizing confidential entries, updating files regularly, and including contact information for questions. Buyers should verify before purchasing by checking sellers.json routinely, investigating discrepancies, and prioritizing transparent sellers and platforms. Both sides should collaborate by reporting issues to platforms, sharing findings with industry groups, and supporting continuous improvement of transparency standards.

Frequently Asked Questions

The IAB Tech Lab doesn't mandate specific update frequency, but best practice suggests updating sellers.json whenever sellers join or leave the platform—ideally daily or at least weekly. Frequent updates ensure buyers have current information and demonstrate commitment to accuracy. Some platforms update continuously via automated systems. The version number and last-modified HTTP header help buyers determine file freshness.

While not legally required, sellers.json has become an industry expectation. Platforms not publishing sellers.json face buyer scrutiny about transparency commitment. Major DSPs may reduce spending or avoid platforms lacking sellers.json. Industry reputation suffers when platforms resist transparency standards. The competitive disadvantage of not publishing often outweighs any perceived benefit of opacity. Most reputable platforms now publish sellers.json.

Sellers.json makes unauthorized reselling visible but doesn't prevent it automatically. Platforms could still list unauthorized sellers. Sellers could misrepresent themselves. Confidential entries could hide bad actors. However, sellers.json combined with ads.txt creates verification mechanisms exposing unauthorized activity. When buyers check both standards, unauthorized reselling becomes detectable. The accountability and transparency discourage fraud even if they don't eliminate it entirely.