I bought a BL-5J from Amazon, marked as genuine. It was anything but. Looked -very- convincing though. Threw it out as soon as I realised it was fake - wasn't worth the postage to try to get a refund, and I wasn't about to risk my phone.

If you buy things like batteries and memory cards from an Amazon reseller instead of directly from Amazon it's just as bad as buying from eBay.

Problem is it's very hard to tell whether a third party battery is good to use or not - many manufacturers take shortcuts when it comes to various protection circuits and whatnot. Just not worth the risk on such an expensive item as the N900.

I'd look to the price as a big clue - if it's too good to be true, it most likely is.

- Micha.