I have used ebay batteries (and chargers). If they're OEM (Apple brand) then you just need to worry about the sellers reputation. Also, many are refurbished but still OEM which I wouldn't touch. Just make sure of what you're getting. Mine have been OK but I guarantee that one for $60 is crap. That said, my philosophy is now as follows: I paid a lot of money for my mac. I'm not going to scrimp just to save a few bucks on something that may damage it.
Pay the extra extortion fees for the real thing from a reputable dealer even if it's Apple.