BE VERY CAUTIOUS
The official free upgrade went away many years ago. What remain are mostly MALWARE versions that have poisoned variants of the upgrade installer.
I DO NOT recommend trying to get a "free" upgrade to Windows 10 any more, unless somehow you can verify that the installer is the original one from Microsoft. All others are very suspect.
And you still have to activate the fake upgrade somehow. It can be done, but again, BEWARE of sites claiming to have "activation codes" -- they are mostly MALWARE and can infect your computer just by hitting the webpage.
So are the ebay $30 upgrades mostly legit?