buying used bike and looking at battery options

I am thinking of buying a used bike but am worried about the battery.The bike retailed for 1800.00 and I can buy it for a fraction of that but I`m worried about the battery.To buy a battery from the dealer is over 800.00 but I see them on ebay for half that.. The battery I need is a 36V 10 Ah Li Ion. I was wondering (other than the size and shape) what should I find out to ensure that this battery will work with this bike and the charger.Any insight would be greatly appreciated thanks