Poor service

I bought a Mercedes and I noticed battery warnings. I told the salesman: Owen, He promised to have it sorted but came back to tell me that their technician says the battery is fine. He advised that if the problem persist I should go to the nearest battery store and check it. I did so and informed him that the battery is dead. Submitted all results and invoice as per request and he promised that I will be reimbursed. Now there’s been no word or feedback and I contacted their customer care to complain but all I got was a reference number and a promise to call back but they never did. I called and WhatsApp’d but all I get is that an agent will Call you back. I’m not getting any help from the salesman either because he doesn’t have his tablet and can’t access His emails. HOW DO YOU SELL A CAR WITH DEAD BATTERY???! Shouldn’t there be checks as you make sales?