There is no way to be certain if the battery you buy from eBay really is from a returned, faulty phone. Does better English skills, a more personal writing style, a 99.3 instead of 98.3 rating guarantee that it's not a counterfeit battery? Or a higher price than the obvious fakes? Not at all.

Personally I prefer to buy products with a 3rd party brand rather than buying brand products suspiciously cheap. If they have a brand, some sort of brand pride exists on some level.

Also, importing fakes is illegal.