iPhone 4 Wont Charge, however the phone appears in iTunes

Hello,

I have an iPhone 4 that wont charge, I bought this phone off of eBay and everything apart from this works. I took the back off it and checked the dock connecter, disconnected it, made sure it was clean etc. Once I reassembled it I plugged it in to test it. The apple logo appeared and the phone switched on, proving that the charging port was fully working. I assumed that it was the battery that was faulty, so I replaced it, it still doesn't work.

Any help would be greatly appreciated. Thank you.