Ebay is a minefield, and Amazon doesn't even sell them - it only "fulfils" them on behalf of 3rd parties (it means the 3rd party has their own stock in the Amazon packing facility).
Having gone the Amazon-fulfilled route I got a decent-looking battery but which performed badly and had no NFC. On peeling the label off it was obviously shonky.
Has anyone had success and verified it was genuine (as far as you can anyway - there are various youtubes and blogs on the matter)?
The zerolemon/mugen alternatives look as if they fail badly after 6 - 9 months so I'm not considering them.