I've bought what seems to be a brand new, "genuine" OEM Apple battery from an eBay seller @ £43 (including postage) who had great feedback (after much research from various sellers).
Well, I just joined the eBay battery club x2 and it was a very good experience. I had said previously that OEM batteries sold on eBay at could be older stock and not really as good as you're expecting. I was wrong (at least with the seller I went through).
The pricey NuPower battery that I bought for my wife's 2012 Unibody MBP started to expand and as it expanded it started to perform erratically leading to premature shut downs and the trackpad not working because the battery was pressing up on it. The NuPower was about 2 years old.
My wife only uses her computer at home and didn't really need a robust battery so I wasn't going to get a NuPower again. Even if I got a 4 year old OEM battery that could only be counted on for 80% of its normal strength, it'd be fine. It only needed to last an hour.
I got a replacement from this seller:
https://www.ebay.com/usr/huguinc
It cost $47.50. It's only $10-$20 more expensive than the non-OEM batteries. I wasn't expecting much, but I got a battery that had a manufacture date that was about 6 months ago and it definitely looked like it was an Apple battery. Coconut Battery reported a design capacity of 97% of it out of the box so it's possible it's B stock, normal variation, or random fluctuation in battery readings.
I was encouraged by that purchase so I decided to also make a long overdue battery replacement in a 2009 MBP that I have as a backup/loaner machine. The battery for the 2009 was only $40 and again had every appearance of being OEM and had 100% design capacity out of the box. The manufacture date was about 18 months from the date that I got it.
I havent' soured on NuPower batteries. I've bought a good number of them over the years and this is the first one to crap out prematurely on me, but if recently manufactured OEM batteries can be had for 1/2 the price, there's no reason to go that route.