iPhone only powers on when plugged in but wont boot.

I purchased a faulty iphone 5 from ebay that the seller said was displaying an "error 21" message when connected to itunes. The phone only powers on when connected to a power outlet or pc but only displays the apple logo for a few seconds and then "connect to itunes". When i try to restore and update in dfu mode i get "error 2009" and 1 time in got "error 21". I have connected to power outlet with no battery in the phone and i get the same "connect to itunes" I have replaced the battery and still no luck. From what i can see there are no signs of liquid damage and no damage to cables/connectors inside. Could it by any chance be the dock connector?