Bought one from eBay and I'm pretty sure it was genuine because the client I replaced the battery for had no issues for it.
Usually what happens in some cases is that the less popular brands have a significantly lower chance of having aftermarket bad quality parts in the market.
I've seen this is the case for Huawei / Nokia and One Plus One.
How do you know they are all fakes? Have you actually tried one?
I've only had issues with refurbished / aftermarket batteries from Apple and Samsung phones.