I wish i had never bought a phone from this compan...

I wish i had never bought a phone from this company.
I bought what was supposed to be an 'A grade iPhone 6' and there is a blue colour mark on the screen whenever there is a white background.
EVEN WORSE THAN THAT if i am on an app for more than a minute, the phone turns itself off and when i try to turn it back on it says the battery is low, when it was full charged just before turning itself off.
DO NOT BUY FROM HERE!!!