A reasonable question: if Closo runs on servers, why does it need software on your computer? Why can't we just call a marketplace's API like every other web app does?
Short answer: most resale marketplaces don't have a listing API at all. The ones that do (eBay, Shopify) have rate limits and bot defenses that punish heavy users. To list reliably across all 6 marketplaces, we use your browser instead — because your browser is the only piece of software that's allowed to act as you.
The problem
Poshmark, Depop, Mercari, and Vinted have no public API for listing items. There's no documented way for a third party to say 'create this listing for this seller.' If you want to list there, you (or something acting as you) must use the marketplace's website.
eBay and Shopify do have APIs. But eBay limits sellers to a few thousand API calls per day and forces token refreshes; Shopify charges per call past a threshold. Bulk crosslisting across both gets expensive and slow fast.
Worse, when a service like Closo tries to talk to Poshmark or Depop directly from a data center, the marketplaces detect that within seconds. They look for traffic from cloud providers, unfamiliar IP ranges, and TLS fingerprints that don't match real browsers. Cloudflare, Akamai, and PerimeterX (the three bot-defense systems most marketplaces use) block server-to-server requests almost immediately.
How an extension solves it
The Closo extension lives in your browser, uses your session, and sends from your IP address. To the marketplace, the request looks exactly like one of your normal browsing actions — because that's what it is.
Behind the scenes, the extension opens hidden windows (sandboxed tabs Chrome calls 'offscreen documents') and runs the listing flow there. You don't see them. While you're scrolling Instagram, the extension might be filling a Vinted listing form for you in a window that lives off-screen.
Because the marketplace sees normal behavior — your IP, your cookies, your browser fingerprint — your account stays healthy. We've spent a lot of engineering on making sure the extension's traffic looks indistinguishable from your manual activity.
The tradeoff
An extension-based approach has costs, and we won't hide them:
You must keep Chrome open
First-time setup takes 3-5 minutes
Extensions need permissions
Compared to other tools
Some crosslisting tools try to skip the extension by listing entirely server-side. These break frequently — when a marketplace updates its bot defenses (which happens monthly), the server-side approach goes dark for days while the vendor scrambles to patch. Sellers stuck on those tools wake up to broken queues.
Some other tools require you to forward your phone screen or run a desktop app that mimics a browser. Those work, but they're heavier and feel intrusive. A browser extension is the lightest-touch option that still gives us real reliability.
Install the extension from the Chrome Web Store, then come back to Closo. Your first crosslist takes about 90 seconds end-to-end.
Connect a marketplace