martyls :
Bought Windows 7 install disc from e-bay. It has no key to install. How do I get key from disc ? Is it useless ?
You just gave some random person an extra 5 or 10 dollars.
The license is what costs. Given a valid license key, you can download the ISO directly from Microsoft for free, and create your own install DVD or USB.
For future reference, buying licensed software from ebay is rarely a good idea.