If you're running 10.6.4, then odds are all those battery updates have already been rolled into the OS.
But I'd have to say that it sounds like the seller was feeding you a line about installing a new battery. If they actually did install a new battery, then that would mean that there's probably something wrong with the logic board that will now bite you in the rear some day down the road.
I recently purchased a Macbook from eBay and it all works fine and I knew when purchasing it that the computer would only work when it is plugged into the wall, the seller said he had put a new battery in it before selling but the problem was still there.
I realised soon after receiving the Macbook that there was no Battery Update bundle installed so did that straight away (Battery Update 1.2), I have tried to install Battery Update 1.4 but whenever I do so it tells me that the update will only work on OS X Tiger 10.4.11 or newer or OS X Leopard 10.5.8 or newer, and since I am running 10.6.4 I don't understand why this update won't install.
Anybody know of any other problems that could be causing the battery to just not want to charge at all?