Terrible services

I bought a mac book at Istore in Sandton city and the unit was water mark damaged. I returned the unit in less than 7 days of purchase, and they claim I ruined the unit. When I had bought it with a water mark damage. Now they’re expecting me to part with money equivalent to a new mac book, to “fix” a unit which I already bought with the damage. I am unhappy with apples services as even their international helpline couldn’t account for selling me a damaged unit.