ebay has been overrun with "genuine fakes". Although they are not technically BAD, the voltage detection is almost always WAY off and very problematic, which leads me to believe the battery could harm devices with long term use. I bought three batteries for my HD2, one which I knew was a non genuine, two which I thought were genuine and only one turned out to be, but it was purchased from a US seller who sold them as genuine or full refund, taken from actual HTC phones.
I even bought a battery for my samsung digital camera on ebay once, guy said "GENUINE, NOT FAKE, SAMSUNG OFFICIAL", even sent an original (see genuine fake) box for proof. Popped in the battery and lo and behold: FAKE! Guy offered a full refund and said to ship battery back. Which I never did, because shipping back to China costs twice as much as the item you buy.
You reaaaaally have to be careful buying batteries, flash memory or any electronic for that matter off ebay. Crooks everywhere. Especially when they ship from China!