A few minutes of one-time setup gives ChatGPT real secondhand-marketplace search — including Deep Research support.
ChatGPT supports custom MCP connectors through developer mode. Setup is a one-time flow from a desktop browser; once connected, you can invoke Secondhand MCP in any chat, and its search/fetch tools work as a ChatGPT Deep Research source.
Takes about two minutes, once
In chatgpt.com → Settings → Apps & Connectors → Advanced settings, toggle Developer mode on.
Back in Settings → Apps & Connectors, click Create.
Name: Secondhand MCP
URL: https://secondhandmcp.com/mcp
Description: Search secondhand marketplaces
Authentication: OAuth
Click Create, then complete the sign-in flow (email magic link or Google). This creates your free account.
In a new chat, click + → More → select Secondhand MCP, then ask for what you want to find.
Secondhand MCP exposes the standard search and fetch tools ChatGPT Deep Research expects, so you can add it as a Deep Research source and have ChatGPT compile multi-marketplace price research — for example, "research the going rate for used Eames lounge chairs across marketplaces."
Custom MCP connectors are a developer-mode feature in ChatGPT. It is a one-time toggle in Advanced settings — everything after that is normal ChatGPT.
Yes. The server implements the standard search/fetch tool contract Deep Research expects, so it can be used as a research source.
No API key for Facebook Marketplace, Depop, or Poshmark. eBay optionally uses your own free eBay developer keys. Authentication to the service itself is OAuth — you just sign in.
Yes — 10 searches a month, no credit card. Pro ($5/mo) raises limits and adds Depop and Poshmark.