Use Claude to Check Usernames and Compare Reselling Fees

You're about to rename your shop, so you ask an AI assistant whether "thriftlane" is free on Poshmark and eBay. It answers confidently. It also has no way to open either profile page, so that answer is a guess, and a wrong "available" is the kind of mistake you find out about after you've reprinted the labels.

An assistant can only be as accurate as the tools it can call. Username Inspector now runs a free MCP server that gives it two: a username availability checker for 12 reselling platforms, and a fee comparison for 7.

What the MCP server gives your assistant

MCP (Model Context Protocol) is the standard that lets an AI assistant call outside tools instead of answering from memory. Username Inspector's server exposes two read-only tools, and neither changes anything on any platform:

• check_username looks up one username or store name. eBay, eBay Store, Poshmark, Grailed, Vinted, Whatnot, Bonanza, and Chairish are looked up directly. Poshmark, Vinted, and Bonanza usually return a live answer, while eBay, Grailed, Whatnot, and Chairish often block automated lookups. Depop, Etsy, Mercari, and Ruby Lane always block them, so for those the tool returns a profile link for you to open.
• compare_fees takes a sale price and works out fees and payout on eBay, Poshmark, Mercari, Depop, Etsy, Grailed, and Vinted, using the same rates as the fee calculator.

It's free, needs no account or key, and has its own reference page with the limits and privacy details.

Connect it in a few minutes

The server address is https://usernameinspector.com/api/mcp. How you add it depends on your assistant:

• Claude Code: run claude mcp add --transport http username-inspector https://usernameinspector.com/api/mcp.
• Cursor and other clients with a JSON config: add the URL under mcpServers with the key url. The MCP page has the exact snippet to copy.
• Clients with a custom connector or remote server option: add a new server and paste the URL.

After that, ask your assistant what tools it has. You should see check_username and compare_fees.

Check a username

Name the platforms, because a short list gets a faster, clearer answer:

The assistant calls check_username and gets one status per platform:

• Available: no profile exists with that name.
• Taken: a profile exists, and the result includes the link.
• Unverified: the platform can't be checked automatically, or it answered with a bot-protection page. Open the returned link and look yourself.
• Error: the check timed out. Try again in a moment.

In our own testing from our servers, eBay, Grailed, Whatnot, and Chairish often answered the lookup with bot-protection pages. The tool reported those as unverified, with a link, and Poshmark, Vinted, and Bonanza returned real answers.

Two cautions apply to every availability checker, ours included. An available name isn't a reservation, and a platform can still refuse it at sign-up if it's reserved or blocked. Availability also comes from the public profile page, so confirm on the platform when you register. Our guide to what to do when your username is already taken covers the next step when a name isn't free, and the platform username rules post covers length and character limits.

Compare fees on a real sale

Give the assistant the whole picture, not just the price:

For those numbers, compare_fees returns this, best profit first:

Vinted and Poshmark ignore the shipping you charge and the postage you pay, because the buyer pays the label on both, so their rows aren't directly comparable with the others. Depop's 3.3% plus $0.45 and Vinted's zero seller fee look hard to beat on paper, but fees are only part of the picture. eBay has far more active buyers than Vinted, so an item can sell in days on one and sit for weeks on the other. Our guide to reselling fees compared across platforms covers that tradeoff.

You can try your own numbers right here without an assistant:

For one platform in depth, see the fee guides for eBay, Poshmark, Depop, and Etsy.

Prompts that work better

• Check one name at a time. The server caches a result for five minutes, so repeating the same lookup returns the same answer, and it's rate-limited to about 12 tool calls a minute. For a list, go to the website instead.
• Name the platforms. "Poshmark and Bonanza" gets a tighter answer than "everywhere."
• Give every cost. Item cost and postage turn a fee table into a profit table, which is the one that decides where to list.
• Ask what it couldn't verify. "Which results should I check by hand?" makes the assistant call out the unverified rows instead of skimming past them.

Limits and privacy

• Rate limit: about 12 tool calls per minute per client. Past that the server answers with HTTP 429, and a well-behaved assistant waits instead of retrying in a loop.
• Four platforms are links only: Depop, Etsy, Mercari, and Ruby Lane. Their bot protection blocks automated checks, so no honest tool can give you a live answer there.
• Fees use each platform's standard US rates and exclude sales tax, so treat them as estimates. Every result carries the date its rate was last verified.
• Your usernames: the assistant sends the name to our server, which looks it up on the marketplace and returns the result. We don't store usernames or write them to our logs. The privacy policy has the details.

When to use the website instead

The MCP server suits a quick question inside a conversation you're already having. For anything bigger, the website is the better tool: manual mode checks up to 500 usernames, exports a CSV, and opens every profile in its own tab. The step-by-step guide covers both modes and the fee calculator.

You can also run a single check without an assistant on the home page checker, then add the server later if you find yourself asking the same questions every week.

Frequently asked questions

Can Claude check if a username is available on eBay or Poshmark?

Not on its own, because an AI assistant can't open marketplace profile pages from inside a chat. Once you connect the MCP server, it can: check_username looks the name up on eBay, eBay Store, Poshmark, Grailed, Vinted, Whatnot, Bonanza, and Chairish (Poshmark, Vinted, and Bonanza most reliably), and returns profile links for Depop, Etsy, Mercari, and Ruby Lane. Platforms that block automated lookups come back as unverified with a link.

Is the Username Inspector MCP server free?

Yes. It needs no account or API key. It's rate-limited to about 12 tool calls per minute per client so it stays available for everyone.

Which AI assistants work with it?

Any assistant or agent that supports remote MCP servers over Streamable HTTP. The MCP server page has setup steps for Claude Code, clients that use an mcpServers JSON config such as Cursor, and clients with a custom connector option.

Are the fee comparisons exact?

They're estimates for US sellers using each platform's standard rates, and they exclude sales tax. Each result carries the date its rate was last verified. Check the platform's own fee page before you price something.

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