probably something apple repair people know about em, like the more recent macs where the nvidia graphics would fail all the time and they refused to acknowledge it until way way way way late. usual apple stuff.
but if it doesn't start off the charger without a battery, don't waste money on the battery. could also hot wire a spare battery off another laptop too, unless apple has some proprietary chip there to give OK that battery is theirs.
unless it's for scrap, i wouldn't trust ebay batteries, most likely protection circuit wont be up to scratch either among the other issues mentioned above. Don't know exactly what they use but if there isn't some proprietary handshake, which there might well be, you can simply hotwire a battery off something else just to make sure it charges and runs off it.
more importantly don't sink money if you're not sure it turns on or charges/runs off battery if you can help it...