Please help! Sony Xperia Z problem!

[Moadperia]

I just bought the Sony Xperia Z 16GB c6603, i had this problem out of the box... 
It keeps shutting off when i apply a little pressure on the back of the phone or at the middle of the front screen.
i guess it's an  hardware problem. but what could it be ? any suggestions how to fix it ? 
sorry if my english is kinda bad!

[Uliwooly]

does this happen without the SIM card? 

if you bought it new just take it back to the store, if you bought it from someone else, return it ASAP (as soon as possible)

[Moadperia]

Yes it does happen without the SIM card. 
and unfortunately can't return it now, so is there any fix ?

[Uliwooly]

That's a hardware issue, you will have to contact your Local support or if you bought it used, contact the seller 

[Moadperia]

Thank you ~uliwooly, and yeah i bought it used like new. and the seller wont reply back so i guess i'm stuck with it... i have a minimum backgroud on fixing phones so what could the problem be and what can i do to try to fix it?

[Uliwooly]

@Moadperia

it could be a loose SIM tray, it could be a faulty motherboard, etc.