RealX402

Is this x402 seller legit before you pay it? A pre-payment check for a marketplace with no real vetting of its own — no signup, pay per call.

The problem

The x402 Bazaar and Agentic Market list 14,000+ self-declared services. Anyone can list one — there's no vetting before it shows up, and no independent conformance testing exists at the protocol level either. A well-formed, signed 402 challenge isn't the same thing as a real, working service behind it.

What it checks

Given a seller's resource URL, /verify/seller probes it live (no payment made) and checks:

Exists — does it actually respond with a well-formed x402 402 challenge, right now.
Declaration consistent — does its own declared example output actually validate against its own declared schema. Catches broken, copy-pasted, or abandoned listings.
Payout sanctioned — is its payTo address on the OFAC sanctions list, checked via Chainalysis's on-chain oracle.
Wallet history — how old is the payout address and how much activity does it have (Base addresses only) — a bonus signal, not part of the verdict.
Cairn citation — if the exact seller you're checking has a published Cairn conformance report, its verdict is included as supplementary evidence — a dated, independent finding, not a live check, and never part of our own verdict.

Honest about what this is: confirmation that the checkable things about a seller check out — not a guarantee it's trustworthy. No free source can promise that today. This is a first layer toward that, not the finished answer — every call, and the support behind it, helps build toward it.

This route is itself checked out: an independent, signed conformance report from Cairn — an autonomous agent that runs real adversarial payment batteries against live x402 sellers — passed all 16 checks against this endpoint. The two checks are complementary: this one answers "does this seller check out" on every call an agent makes; Cairn's answers "does this seller's payment handling actually hold up," run once and published permanently for anyone to verify.

Call it

curl -i https://goodsong.dev/verify/seller \
  -H "Content-Type: application/json" \
  -d '{"resource_url": "https://api.example.com/paid-endpoint"}'
# → 402 Payment Required, with the x402 payment challenge in the response body

Pricing

$0.005/call

Pay per call via x402 (USDC on Base) — no account, no key. A failed check (our source was down) is always free. A flat-quota API key is also available if you'd rather not pay per call — same account as goodsong.dev's other verification endpoints (citations, packages, repos, legal case citations).