It really is a "buyer's beware" market. Consider it an electronic garage sale or "Dollar Store". I shop at both sometimes.
Over the years I've bought 100's of items down by dBay and I've never had any serious problems, and the couple of hic ups I've had have been rectified by the seller.
I've bought electronics, video games, many many hand-crafted teddy bears (my wife's passion) and stuff for my other passion ~model railroading. And several gadgets for my camera.
So far so good.
As for quality, much of the cheaper stuff is "Dollar Store" quality, but for those of us on a budget we do shop at the dollar store and while that gift bag for 50 cents is not the same quality as the Hallmark one for $4.00, it does the job just as well and when I'm handing out two dozen Christmas presents at my staff's Christmas Party, that's $42 in my pocket and not Hallmarks.
But the brand name stuff I've bought seems to me to be exactly the same as the original. (Maybe it is "seconds" but I've never noticed??)
Camera stuff I've bought include 'star filers' - a novelty item that put me back about $10 for four. a flash diffuser, a wireless shutter release (Yongnuo - works great), step up rings, replacement strap, and the list goes on. I did buy a brand name Kenko Polarizing filer, for way less than half the price they want for the same thing at the camera shop, I've used it, seems to work as it should, tried the test on the video monitor and yep, turn it right and you see the monitor, turn a bit more and you you see a black hole. I lost the lens cap on my camera. Could not believe the price at the camera store. Bought half a dozen for less than half the price one would have cost me, it works, it says Nikon (unlike the camera store which was a generic). I can't tell the difference. Same thing goes for those little hot-shoe covers, I bought a half dozen with a wee level bubble in them. Work like a charm, cheap like borscht. Stuff like that, go for it.
As for less than honest eBay seller ~ that may be true but I've not had that problem. Once I've had good service from any seller they go into my trusted seller list and that helps.
Keep in mind I'm comparing these eBay bargains to our heavily over inflated Canadian prices. I'm sorry but the price difference is just too much to pass up. And I live 3 hours from the city and only go there a few times a year so if I want something mail-order is one way I can get it faster. That may not be the case for my urban friends who can pop down to Best Buy or a big camera store for their needs.
Having said all that, I did not buy my camera on eBay. Again I think of it like shopping at the Dollar Store, cheap but not always top grade products.
That's why I hesitated on buying cheapo batteries. I was concerned that a cheap battery with poor quality control may put out the wrong voltage, watt or whatever which may damage my camera, and I did see lots of comments that they did not work. That may have been fixed now I'll be watching to see how the OP's decision works out for him.
Like the saying goes, Buyer Beware and if you are comfortable with the risk, go for it. Like my papa taught me, never buy a parachute on sale.