Sumsung J5 Prime

I bought A cell phone at Donna-Claire in springs mall on Sunday, the phone was supposed to be a dual sim and the person who was helping me at the shop told me that if there is a problem with the phone I must take it back within 7days, so the phone has two slots the small slot the sim fits perfectly and the second slot has a big tray for the memory card and sim card but the sim card doesn't fit perfectly then I decided to take it back so that they will put in for me but service that I get there was not good, they couldn't help me instead they say I must take the phone to the sumsung store, and they told me that the box is no longer in good condition because I've written something on it, what I was looking for is for the problem to be solved its not my problem that they gave me a faulty cell phone so I'm not the one who move up and down for the factory faulty what I want is my phone back or my money. The 7 day is not over yet.