iPhone 4s stuck in DFU mode and an error 21

Hi guys. Here's the deal. Recently, I bought an iPhone 4s from Amazon, knowing it was stuck in DFU mode, I bought it because it was cheap and I thought it would be easy to fix. There has been nothing on the screen that's shown up at all. I've plugged it into iTunes, nothing, tried all the power and home button combinations on the internet but to no avail. iTunes recognises it because when I plug it in it starts up, however I can't restore it because of an error 21. WHAT SHALL I DO? Should I just cut my losses and take it to an Apple shop or phone repair shop? This is really bugging me so any help is much appreciated.

Thanks