To be honest, I'd be more inclined to go with an ebay vendor who does NOT claim it's a real OEM Samsung battery, and who's honest about the fact that it's a generic battery. Let's be real. Any battery that claims to be genuine Samsung and costs less than the battery's cheapest known wholesale cost is guaranteed to either be a) fake, or b) a defective reject that Samsung intended to destroy rather than sell.
IMHO, there's no need to distinguish between "fake" and "unauthorized but made in the same factory", because as a practical matter, a factory that does an unauthorized second run is going to cut the same corners with that second run that it would have cut with outright generic batteries anyway. A battery isn't a complex electronic device with long assembly-line setup time. It doesn't take long to have someone swap out the thermister reel for a reel of resistors that look like thermisters, or to dial down the electrolyte dispenser to use a little less than normal, or swap out a reel of high-quality Japanese capacitors or resistors with 1/2% tolerance for a reel of the cheapest 20%-tolerance Chinese capacitors or resistors you can buy for a fraction of the price.
For the most part, buying a pair of generic batteries + charger with free shipping for $10 from ebay is kind of like playing roulette and betting on red or black. Just about everyone who's bought a pair from someone on eBay has ended up with at least one battery that's almost as good as the one that came with the phone, and most end up with two useful spares.
Occasionally you'll hit the jackpot and end up with a battery that seems to be better than the one that came with the phone, occasionally you'll get two batteries that are junk. Take them for what they are -- cheap spares that are handy to have until the day we can buy a real extended battery that can handle a good, hard 16-20 hours of aggressive pocket-laptop use, and maybe a full day of use as a conventional phone.
I personally bought a pair from a guy (I assume) named "leiiho", and I'm perfectly satisfied with them. Would a genuine Samsung battery get another 100mA or so of real life? Maybe... but for the difference in price between what I paid and what a genuine Samsung battery would cost, I'm happy to spin the roulette wheel and bet on red (or black).
For extended batteries, I'm a little bit pickier and tend to lean towards Seidio for my real daily-use battery, but when the $12-15 extended batteries hit eBay, I'll probably buy one of them, too, just to have around in case we have a hurricane next summer.