Fair warning some people get ego Batterys at fasttech and love them. The 2 900 mahs I picked up have not been a good experience. Basically you have to be extra careful not to over tighten atomizers or chargers otherwise you will be constantly adjusting the center pin of the battery. Worse the center pin is pretty loose fitting and it is very easy to destroy the connector all together. 6 months in 1 battery = destroy and the other needs adjusted at least once a week even with me being careful. The 2 I have are fiddly at best.
Contrast that with my 2 genuine joyetech ego batterys which I have never needed to adjust despite using the same toppers and chargers.
Not knocking the fasttech ones as long as the buyer knows what they could be in for.
As with also buying things on ebay... I think you just need to be careful of WHAT you are buying. I expect that if I buy a knockoff its just not going to be as good as the real thing, most of the time anyway. Fasttech does have genuine things on there as well. I'm really trying to not waste money when it comes to buying vaping equipment, by not wasting money that means that I have to be willing to spend just a little bit more on authentic equipment. I'm a believer of you get what you pay for in the world of electronics. Not to say there aren't good deals out there.