I have done that before, but the key only works for minutes, and ebay I am sure has removed these non-legit service. I needed 3 keys, and this time I invested in what I thought were legit services.
One other thing you might try is to download the most recent Windows ISO and try to install from it using the key.
You can also try to troubleshoot by using the telephone activation process if you can get someone to help you with it.
You should be able to access it by going to Start->Settings->Update and Security->Activation. There should be an "Activate by Telephone" option there.
Years ago, I bought an OEM boxed version of Windows 7 Pro for a new build on eBay. It refused to activate via the online process no matter how many times I tried, to the point where I was convinced it was a counterfit.
However, it activated via the telephone process just fine. It did the same thing the several times I subsequently had to reactivate it on the same hardware after a system wipe and clean reinstall. I did that up until I found Advanced Tokens Manager which allowed me to back up/restore the Windows activation. Never had to worry about it again after that.
That Win7 product key was eventually used to upgrade to Win10 and is still in use today on that same old hardware.