Can I use the Windows key from a notebook that I have purchased?

I bought a notebook on eBay. There's a Windows key on the bottom. Can I use the key?

In principle yes. In the case of newer notebooks, the key is permanently stored anyway, so that you don't even have to enter it when you reinstall.

I bought a notebook on eBay. There's a Windows key on the bottom. Can I use the key?

You can and may of course use this product key for the device you have purchased. The license is linked to the notebook through the affixed certificate of authenticity. May only be used for this. It may also work with modified hardware, but reselling it would be illegal.