Won't allow me to restore it in iTunes

I bought an iphone 3gs off ebay yesterday. I was told from the owner that it had been jailbroken. He had put it in recovery mode to remove all of his stuff of the phone

When I went to plug it in to restore it in iTunes, it recognises the iphone and comes up with the usual message , "iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes."

I then proceed to restore the phone and it comes up on the iphone with the apple logo and a loading bar directly underneath it.

It gets to the firmware section of the restoring process in itunes and then fails , quoting error (-1). (this has happened approx four times)

Now, I am a complete idiot when it comes to technology and when I have tried to google my problems the solutions are far too technical to understand and I am basically at a total loss !!! any help would be seriously appreciated !!