You can buy grey market keys off ebay, but that's the only way you're getting a $10 license.

Just that - Win 10 Pro keys are dirt cheap from many online key sellers - and just work. I suspect that and OEM sales are how 99% of windows licenses are now sold - unless you know of people who buy them in a box from a store. Was a little bit 'worrisome' before that MS might clamp down - but the last decade of 'free updates' and 'Win 10 will just evolve' indicates their thinking. Windows is near-as-damnit a free platform (which MS can then use as upsell from)

And now that you can just install windows 10 pro and click "nah, I'll give you the license code later", many people won't even bother. As far as I can tell, the message in the corner is the only issue without activation.

Windows 10 is pretty much shareware at this point. You can download the full version for $0, just with features knocked out until you pay to "register".

Those are volume licenses that are given to microsoft partners for mass deployment. At some point those keys can/will be revoked when they reach a certain number of activations.

If the 4 dollars I spent on ebay lasts a couple of years (it already has) its easily worth it. I really don't want to spend 200$ on a pro license.

IIRC European countries can sell the OEM licenses that come with their computers.

This means you can easily get a Home or Pro license from resellers or ebay for around $20