Unable to use Mac OS X Install DVD

My iMac came with Mac OS X Install DVD, but since I have upgraded to Mountain Lion through the Apple App Store and have made a bootable USB. So after formatting my HDD I'm unable to boot from the USB or the DVD. When prompt to select "where you want to install OS X" from the Install OS X menu I am unable to select the device (This disk doesn’t use the GUID Partition Table Scheme), Also I have tried using the factory DVDs but nothing happens, after loading the DVD and holding down the "Option Key" (Alt) the DVD then gets ejected. I have run out of ways to correct this problem, and I have sold this computer online and need to ship soon, and your immediate response is greatly appreciated!