I know you can buy replacement keyboards, that's for sure.
In answer to your question, If it relates to how long the laptop can run off the battery, then that is answered by the WH rating (6600 mWH, 8300 mWH, etc). If your question is talking about how long the battery will continue to hold a charge, before dying on you, then that's a different question. EBay is gambling, pure and simple. I would find out any information about the battery, before going forward. Try to find out any information on the company, the model-number of the battery, etc. Anything.
Though, as a consolation, Dell doesn't manufacture batteries, they buy them from an OEM. So, eBay does offer you a chance to buy them from a retailer, without having to pay Dell in addition. Further, even if the battery has half the functioning life-span of a battery purchased through Dell, you can afford plenty of eBay-bought spares, for the price of even one from Dell.