Username Inspector

What Username Inspector MCP server does

The Username Inspector MCP server gives an AI assistant read-only tools for resale-marketplace research. It can check a username or store name on eBay, eBay Store, Poshmark, Grailed, Vinted, Whatnot, Bonanza, Chairish, Depop, Etsy, Mercari, and Ruby Lane. It can also produce public profile links and compare estimated seller fees on eBay, Poshmark, Mercari, Depop, Etsy, Grailed, and Vinted.

The service is intended for people choosing marketplace identities or estimating what remains from a sale. Fee calculations use US rates, exclude sales tax, and include the date on which each rate was last verified.

How it works

The hosted lookup endpoint is available at https://usernameinspector.com/api/mcp. A separate links-only endpoint is available at https://usernameinspector.com/api/mcp-links; it builds profile URLs without contacting marketplaces. Both use MCP over Streamable HTTP.

check_username accepts one username per call and optionally a selected list of platform IDs. Usernames must contain 3–50 letters, numbers, hyphens, or underscores. Results use available, taken, unverified, or error, with a profile link where one is available.

For checks, the service makes ordinary HTTPS requests similar to opening a marketplace profile page. It does not use marketplace APIs, require logins, or attempt to bypass bot protection. A challenge or failed response produces unverified rather than an inferred result. Depop, Etsy, Mercari, and Ruby Lane always return links because they block automated checks; other platforms may also block requests.

Setup and configuration

No account or API key is needed for the hosted endpoints. Claude Code can add the main endpoint with its HTTP transport, while clients such as Cursor can use a JSON MCP-server configuration.

To run the Username Inspector MCP server locally, install Node.js 18 or newer, then run:

npm install
npm start

The default endpoints are http://127.0.0.1:3000/api/mcp and http://127.0.0.1:3000/api/mcp-links. Set PORT or HOST to change the local binding. The API directory is arranged for deployment as Vercel serverless functions. The server is stateless and uses Streamable HTTP.

Tools and capabilities

• check_username: checks one username on up to 12 supported platforms.
• get_profile_links: creates marketplace profile URLs through the links-only endpoint.
• compare_fees: ranks estimated proceeds for a USD sale and can include shipping charged, item cost, shipping cost, and an eBay promoted-listing rate.

When costs are supplied, compare_fees also returns profit, margin, and return on investment. Tool responses declare output schemas. Username results are cached in memory for five minutes per username and platform, while errors are not cached.

Limitations and notes

The Username Inspector MCP server permits about 12 tool calls per minute per client, with HTTP 429 responses and a Retry-After value above that limit. Requests are limited to 16 KB and batches to five messages. Outbound marketplace checks run at most 12 concurrently, with a five-second platform timeout and a nine-second call timeout.

Marketplace behavior can change, so an available or taken result should be treated in the context of the platform's current behavior. Fee rates can change too; verify them against the marketplace before pricing an item. The assistant sends the username to the service for lookup, but the project states that usernames and IP addresses are not logged or stored. MCP_DISABLED=1 disables both endpoints, and MCP_LINKS_DISABLED=1 disables only the links endpoint.