An operating system needs to be installed from the master Windows DVD as it needs to be tailored to the hardware in the computer during install.
All computers have slightly different hardware in them and Windows is tailored during the install to work with that specific hardware.
You cant just copy the Windows code from one PC and expect it to work on another PC.
The other problem is that legally once Windows has been installed on the original PC it cannot then be moved to another PC. This is part of the license agreement for Microsoft "OEM" software used by system builders.
Has your PC got a "Windows" sticker on it showing the serial number of the product (it should be stuck on the outside of the case somewhere)?
If it has not then the person who sold you the PC from ebay has broken the license agreement for Windows OEM software.
It may even be they used a "dodgy" version of Windows.
If you want to put Windows on the new PC you will need to buy the OEM version of Windows 7, which is about £80.
http://www.ebuyer.com/259863-microsoft-windows-7-home-premium-w-sp1-licence-and-media-1-gfc-02050