Quant Agent ShopBuild an x402 Seller on Base
How we built and launched this shop, written up step by step: from an empty folder to a pay-per-call API on Base mainnet that software agents can find in the Bazaar and pay in USDC. Every command, setting and mistake comes from our own launch on 22 October 2026.
- Payment safety the middleware leaves to you: refuse reused and short-lived payments, allow one paid call per payer at a time, stop a job before its payment expires, never charge a failed call, and guard every caller-supplied URL against SSRF, with an offline test for each.
- Pricing above the fee floor: unit cost plus about $0.0011 a call (the CDP facilitator fee plus cashing out), with a script that shows how many paid calls a month your server needs to break even.
- A hardened server for about $6 a month: Ubuntu 24.04, key-only SSH, a firewall that allows SSH only, the app as a no-shell user in a sandboxed systemd unit, root-only secrets, and a named Cloudflare Tunnel so no web port is open.
- Mainnet and discovery: the CDP facilitator with an IP-allowlisted key, Coinbase's keyless validator, one disclosed purchase per route to get indexed in the Bazaar, x402scan registration, a remote MCP endpoint, and the official MCP registry using domain proof instead of a GitHub account.
- The market as we measured it: about $80,000 to $130,000 a month of real service payments on Base across all sellers, 93 percent of a week's dollars going to the top ten, and a median listed seller taking about $0.14 in 30 days.
What is included
- The course, an 85-page PDF (x402-seller-course.pdf): ten modules, a troubleshooting appendix you can search by exact error text, and a guide to the starter template. Edition 2026-10, package versions pinned and tested on 22 October 2026.
- The starter template, a ZIP of 13 files (x402-seller-starter.zip): a one-route seller in Python and FastAPI with fifteen offline tests, the server hardening script, the systemd unit, the tunnel config, an env file template and the first-purchase script.
Both arrive as one ZIP the moment your payment settles.
How to pay
US$99, one payment in USDC on Base, from your own crypto wallet: Coinbase Wallet, or a browser extension wallet such as MetaMask, holding USDC on Base. There is no account and no card payment. The button opens a payment page: connect the wallet, approve the payment, and once it settles the page gives you the ZIP to save. Your wallet only signs a USDC transfer and the payment service submits it, so you need no ETH for gas. The page also gives you a private link that downloads the ZIP again, free, for 14 days, in any browser; reloading the payment page itself asks for a new payment, so use the link instead.
Software agents and x402 clients can buy it too: GET https://quantagentshop.com/course/download answers HTTP 402 with the terms,
and the paid request returns the ZIP. Its answer carries a course-redownload-url header, a private link
that returns the ZIP again for 14 days. A paid request sent with an Idempotency-Key header can also be
resent with the same payment and key in that time to get the ZIP again. A payment that already bought the course,
resent without its key, is answered 409 course_already_bought and not charged again.
Sales are final
Sales of the course are final: access starts as soon as your payment settles, and the private link that downloads it again works for 14 days. If the download does not arrive, email [email protected] with the transaction hash from your wallet's history.
No earnings promised
This course teaches you to build and run a pay-per-call seller. It does not promise that anyone will pay yours, and we make no claim about what you will earn. Listing a route does not bring buyers, and most listed x402 sellers earn cents. Education only; not legal, tax or financial advice. Quant Agent Shop is not affiliated with Coinbase, Circle or Cloudflare.