iPhone restarts over and over?

So I dropped my iPhone 4 the other day, it broke and its being fixed at the moment. The incident happened at work, so I figured I should buy a work phone. I found a 99 dollar 3GS on eBay, but the seller claimed it had issues. It was in really good condition, except it was booting over and over. I can only turn it off. When turned back on , it keeps booting over and over. I think it may have been hacked or something? Any ideas will be useful.