mobile

Hey :)
I recently bought a used G900H (seller said the box was only opened and phone not used but doesnt seem right)
So it looks mint, just like a new one would. Is certainly not a fake one too, passes all the tests, works very fluently and scores 42000+ on Antutu. Just flashed lollipop on it and all is nice BUT :
I tried to use s-beam and generally nfc - toggle turns on and phone thinks it works but doesnt, cant transfer anything, nothing happens at all. Same for IR (infrared), app thinks it works but it doesnt and I looked at IR transmitter through a camera and it doesnt light up as it should when being used. Blue led indicator flashed though.
SD card works but a couple of times a day promts that sd card is blank or unsupported, then eventually starts working again. SOmetimes promts it is safe to remove by itself.
Battery cover was 10/10 but just after what was like 5 times I removed and put it back on to interchange sd cards etc, 4 small cracks appeared on it,a piece of it even has fallen out and left a hole near a side.
I was also having battery drains but disabled some google services and battery is fine (by the way I have checked and the battery is genuine one)
What is happening ? I would appreciate any information