Unhappy, highly irate and stressed out.

Good day, I am highly irate, unhappy and stressed out completely. I bought a iphone 12 Pro for R15k cash on Saturday without knowing its a 2nd hand phone. It stated pre owned I didn't realise at that time what it meant. When I got home I Google pre owned because the phone wasn't looking like a new phone it had marks on. When I realised it was 2nd hand. I took it back the next day which was yesterday (Sunday)explaining them why I don't want it. And would like my 15k cash back in order to buy me a new phone or take a contract out. They informed me they can't give my cash back and I must just take the phone home with me. I was in tears as it was all my money I saved. They promise they will call me at 9am to inform me what they decided they still haven't called, when I called this morning they say they are busy. Im stressed out I just want my money back please.