I bought a battery from an authorized mobile shop and it was like 35 USD and they gave me some free gifts with it.
But then I look on ebay, for 35USD I can buy a huge bag of batteries even if they say that it's bulk purchasing it still just sounds like it's a problem. Realistically the mobile shops get the batteries in bulk anyways, why don't we see them selling for like $1 each? And the obvious thing is that things that come out of Asia is mostly pirated. They have a lot of everything pirated really really well. I have this SE W800i I bought for 40USD. Well now i know why.... the battery doesn't even hold the charge well. The phone is R-Tarded half the time. And the Sony Ericsson logo if you look closely under a light and magnifying glass it says "Somy Ericsson". But they make the letting really blurry so you can't really tell and you would think it's an official W800i.
Just looking at the prices is one give away. Only things you can trust via online purchases is if you look at the price and it's about 10 or 20 dollars cheaper than retail and that's probably safe. But if the price of 1 item can buy you a huge bag of them as if you were shopping for economy sized groceries then you will know that there's a problem.