I am building my second build and wondering if I need to waste 100$ on another bootdrive with the key and are the keys on ebay legit?
Microsoft OEM Software Licensing: Rules & Restrictions:
OEM Software may NOT be transferred to another machine. Even if the original laptop, PC or Server is no longer in use, or if the software is removed from the original hardware, OEM licenses are tied to the device on which the software is first installed...
You can check for yourself by reading your own MICROSOFT SOFTWARE LICENSE TERMS in your current system.
If full retail,
Full retail versions are transferable.
What you are really paying for is the key. Keys are limited to one computer. If you plan on no longer using your first build, then you should be able to reuse that key and deactivate your first build. Otherwise, you will need another key.
Generally no. If it isn't from an authorized reseller, it isn't legit. The cheap ones are often pirated as well.
If you can't afford the key right now, you could look into Linux- Valve has made some great strides with Linux gaming that are worth looking into.