On your computer, there will be a sticker, it will look like this. The product key should be on the sticker. If you can not read the product key off the sticker, you will need to purchase another license. The product key is what activates Windows.
The confusion comes from the separation of Software from Licensing. You do not need a license to load windows to your computer you can simply hit Next when it asks for the COA code and you will then have a 30 day trial of that software.
This is a common Scam on Ebay. Items will state Windows 7 Ultimate Loaded. Note Loaded does not mean licensed.