Guys , from my personal experience, don't buy any batteries online unless you reallyyy sure it is real.
My personal Experience is the Following. ( Although a different phone and brand)
When i was in Shanghai, Searching for LG Optimus One P500 (In china P503), in shops around i could not find a single one that was real, there were all copies , although probable working ok but still not original.
Went to the LG website and search for Official support shops in the city, there were quite a few, so i called one and asked for battery, the place said they have. I went there in person and i buy it , paid 100 rmb ( about 12€ or 16$ at 27/02/2012 rates) for it. Had the wait a bit for the person to fill up the paper and warranty and so on.
Been using it for the last month. SO far is Exactly the same as when the old battery was new.No complains at all.
Note: All the Chinese talk was done by my girlfriend.
For Samsung or any other shops, i suppose it pretty much the same.
Another clue is , never buy anything from Shenzhen area ( south China Area), the probability of being fake/copy is 99%).
So my advice is, try search a support shop/place at your own country and ask them for it.
I hope i was clear.