I would think Bill Gates would have something in place not to let the user load it a second time.
Actually it would be 'let a 2nd user' use the same activation key.
I think you're right. It's called Windows Genuine Advantage (WGA)
If something looks 'too good to be true' - it probably is (from an actual Ebay ad:
Like Windows 7 Ultimate (32/64-Bit) Full Version (Activation Retail Key only) $45.00