iPhone 4S boot-looping, cannot update or restore.

I recently bought an iPhone 4S on eBay for around £7.00, the phone, when plugged into a computer will display the "Connect to iTunes" screen, iTunes will recognise it, however when I try to update it, it tells me to restore the iDevice, but every time, the phone goes to the "Progress Bar" screen, but restarts and boots back to the "Connect to iTunes" screen, a few minuets later, iTunes says, "The iPhone could not be restored. An unknown error occurred (9)". The Phone will not go into "DFU mode". When the device is not connected to a PC it will continuously boot loop until the battery dies. I have tried on two different PC's and the problem persists, I have tried restoring on 3U Tools, but still it does not work, I have been stuck on this for a while and have no idea what to do, can someone Help?