I have used Ebay batteries in my macbook, powerbook, and ibook. The oldest battery being nearly 3 years old and it still holds a surprisingly good charge. I personally have experienced problems with both my macbook and ibook's OEM battery. So for me it seems safer going with an Ebay battery for something like 1/3 to 1/6 of the cost.
Not saying what to do but my experience has been great with Ebay batteries.