A port scanner running on a webpage without the users knowlege is never legitimate.

So the question is what legitimate reason is there for a port scanner running in a web browser with the users knowledge?

I don't get upset if someone opens and closes a socket to my VPS to see if something's there. My VPS is exposed to the internet. If a socket opens, it should be secure anyways. There's the chance nginx has an unknown zero day, but if I wanted to avoid that, I'd firewall it.

Things are a little less nice if you open a socket and start sending data to see what's there, assuming the server doesn't respond with a banner.

That might be a reason for you to port scan your network.

It is not a reason for your website to port scan my network. Especially since your website running inside my browser is inside my firewall.

> and want to run a security audit

Then you use tools designed to run security audits. You don't open a huge security hole in everybody's browser just so you can use a browser to run a security audit.

There's really no legitimate reason for eBay, or any other website, to portscan your computer. There's nothing there needed for browsing their website.

That's not what's happening here.

My laptop is not exposed to the public internet because it's behind a firewall / NAT. This is like going to my house, plugging a device into an Ethernet port on my router, and scanning my internal network from inside my network.

Except instead of them planting a device, all they have to do is get you to navigate to their webpage. They're getting your laptop to do the port scanning for them, and in so doing, they get access to your internal network. The problem isn't port scanning, the problem is NAT busting.