Samsung J1 freezing

i bought my phone in less then 3 days it was freezing, the screen was jumping, the screen had a funny mark inside. <br> i returned the phone as i had the 7 day thing, when i get their the consultant flips through my phone it functions normally he refuses to take it in to get check, telling me if they cannot see it they cant book the phone in. <br> i don't understand my phone has technical issues n because his naked eye cannot see the problem Vodacom wont fix the phone. What am i suppose to do with the thing???