Rechargeable laptop batteries have a limited life, and over time and use the storage capacity of batteries degrade so that instead of 2 hours or more of battery operating time typical of most laptops you get less than an hour of operating time. When it is time to replace your laptop batteries there are a few things which are essential to get from a seller.

1. Get a guarantee of capacity (X mAh) and/or run time performance (Y hours of operating time) - DOA guarantees are pretty much worthless when it comes to batteries as a shot battery generally works, the problem is it works for minutes of use, versus the hours of use it should) - free programs which test the capacity and run time of batteries can easily be found via Internet search.
   
2. Make sure the battery is an Original Equipment Manufacturer (OEM) battery which has not been refurbished or remanufactured - refurbished or remanufactured batteries may initially deliver performance equal to an OEM battery, but over time the performance and capacity of these batteries tends to degenerate much faster than OEM batteries.
3. Ignore guarantees beyond the period covered by PayPal or whatever dispute arbitration you buy under. It is pretty much impossible to tell a refurbished or remanufactured battery from a new OEM battery and when problems show up after you're able to get your money back through arbitration, there is virtually no incentive for the seller to honor their guarantee.

I've bought over half a dozen laptop batteries on Ebay over the past year, and the preceding are my conclusions from my buying experiences. Hopefully this guide helps you avoid the false economy that many laptop batteries offered on Ebay present.