Given it is increasingly difficult to buy locally replacement batteries for cameras and other electronic equipment at any price, plus they are mega expensive, more and more people are turning to Fleabay. It can almost be guaranteed that every “genuine” battery from China is a fake. They can be very convincing fakes but a bit of checking around and comparison with the genuine original will show them up as fakes. Log a protest with Ebay and more than likely the seller will refund the money. Actually, Ebay is getting close to punitive legislative action IMHO as they will fix an individual reported problem but do not appear to take any action against the seller, such as deregistration. It appears that Ebay is willing to profit from the sale of fakes. A quick look at negative ratings and you will see people reporting fakes yet the seller is still in business on Ebay. So, those of you looking for genuine batteries for warranty reasons then Ebay is not the place to find them.
Ok the next trick is overstating capacity. Given that it is illegal to sell fakes more sellers are looking to cream a bit of extra margin by claiming extra capacity. Sitting on my desk at the moment are two batteries for a Canon G12. The Canon OEM is 1050mAH while the clone claims 1500 mAH. Guess which one gives better battery life? As new, my guess is the Canon one if past experience is a guide.
Now testing battery capacity outside a lab is near impossible for the average user apart from the lower number of shots taken and poor performance in cold weather experienced. One reseller does test their Chinese sourced batteries so for an Olympus ZX1 the OEM is 925 mAH, with many claiming even higher, but they claim 700mAH. I believe both the OEM and 700mAH claim as Olympus likely has sourced the best cells while the clone is sourcing the cheapest cells. No issue with that but as for the claims of 1200 mAH? Um? Ebay once again profiting from seller fraud.
But a simple test is weight. Battery chemistry means that all things being equal there is a direct weight to capacity relationship. Yeap, my 1500 mAH G12 clone battery feels a lot lighter than the original one. Big hint that the cell size is smaller so less capacity. Only one warning though on using weight was with bigger video camera batteries the cells can be swapped from L-ion to cheaper NiMh ones which weigh a lot more but have lower energy density so do not work as long. My guess is if more people did this test lead sales to China will increase

Basically Fleabay appears not to take any strict action against sellers engaged in fraud so buyer beware. My approach is nowadays to buy the clearly marked clone batteries that do not claim extra capacity based on the cheapest price with zero negative ratings for the seller. Often the battery will turn up marked as OEM but the seller has not claimed this as they now this not to be the case.
Memory cards used to be the hot fraud area with estimated even by Ebay of over 80% are fakes. I would suggest fake batteries and over stated capacities is even higher.
As for the camera makers demanding original ones be used, well, try and get them! Also cameras plus battery can be the same price as the spare battery with some economy cameras.
Cheers