I don't understand what you all turned on about!
Yes, battery that you bought for 5 bucks is not original, that's for sure, but it's your call to make. You get it cheep for taking a risk that it will not work properly.
I used to own K750 and I got a 5 dollar battery from ebay for it and it worked as normal.
I prefer buying fakes, that don't pretend to be real, so you know what you pay for!
To conclude (THE GOLDEN RULE): you want to be sure - you have to pay more, you want to save buck - you take you chances.