Many of these are OEM or bundled consumer keys that went unused. Some of them will be fine, others may be iffy.
I got 2 Windows 8.1 PRO keys for $60 each, then another 2 for $38 each. A year later, all 4 are still working fine.
Windows 10 keys are so cheap because PEOPLE DON'T WANT IT. Microsoft has only 20% adoption rate in Windows 10, with 50% still on Windows 7. That's why 7 and 8.1 keys are still $60-90, even on the used market.
You should always download the installation media directly from Microsoft. NEVER use a 3rd party ISO as it can be tampered with.