iPhone 3GS still dead after battery and dock assembly

Hi,

I bought an iPhone 3GS 16GB off someone who said it was fully working 2 months ago, it was put in a draw after upgrading, they then came to sell it (2 months later) and it's dead.

I bought it thinking i'll get a bargain, just change the battery and off we go, but it's back fired, and now i think i've just wasted a lot of money!

After taking it apart I noticed the dock water label was pink, so thought I'd buy a new one at the same time as a new battery for super safety. So i've replaced both the dock assembly and battery and the phone is still dead.

I thought it must be a dud battery (it was off eBay), so the seller gave me another, but it's still dead.

I've tried charging over night with different chargers and leads, tried it on the computer, and tried holding the power and home button for over 1min but still nothing! I've been all over the internet and can't find an answer.

Before I stick it on eBay and waste a load of money, I thought i'd see if anyone would have any suggestions.

Thanks in advance