What is Sellers.json?
Sellers.json is a standardized, machine-readable text file that publishers and media owners place on their web servers to transparently declare their programmatic inventory, pricing models, and sales policies. It's the publisher-side counterpart to ads.txt, designed to help buyers understand exactly what inventory they're purchasing and from whom.
Published in 2019 by the IAB Tech Lab, sellers.json works alongside ads.txt to create transparency in programmatic advertising. While ads.txt lists authorized sellers of a publisher's inventory, sellers.json goes further by detailing the publisher's direct inventory offerings, reseller information, and pricing expectations.
Why It Matters for UK Media Buyers
In the UK media buying landscape, where brand safety and viewability standards are increasingly scrutinized, sellers.json provides critical clarity. It helps agencies verify:
- Direct relationships: Whether you're buying directly from the publisher or through intermediaries
- Pricing transparency: The intended pricing models (private marketplace rates vs. open exchange)
- Inventory quality: Which inventory is premium versus standard
- Publisher intent: How the publisher wants their inventory distributed
This is particularly valuable in premium publishing, where UK buyers working with quality outlets like the Guardian, Telegraph, or FT need assurance they're getting authentic inventory at appropriate rates.
Technical Implementation
The file follows JSON format and is placed at /.well-known/sellers.json on a publisher's domain. It contains seller records with details including:
- Publisher name and contact information
- Seller IDs (matching ads.txt entries)
- Inventory types and expected pricing
- Reseller information
- Creation and modification timestamps
When You'd Use It
Media buyers should reference sellers.json when:
- Verifying programmatic partnerships: Confirming a publisher's official resellers
- Auditing inventory sources: Checking where inventory is actually coming from
- Negotiating direct deals: Understanding the publisher's preferred pricing structure
- Implementing brand safety filters: Identifying premium versus non-premium inventory
- Combating ad fraud: Validating inventory authenticity in programmatic campaigns
Current Adoption in the UK
While adoption remains slower than ads.txt in the UK market, major publishers and premium networks increasingly implement sellers.json. This trend is accelerating as agencies demand greater transparency and as privacy regulations make programmatic buying more complex.
Best Practice
Connect Media Group recommends regularly checking sellers.json files for major publisher partners as part of quarterly audits. This helps maintain accurate supplier relationships and ensures your programmatic buys align with publisher intentions and quality standards.