Wrong advice, then refuse refund

I recently bought 8GB ram at Matrix in Bloemfontein at Loch Logan waterfront. On the day of the purchase the salesman said that he wil need to see my laptop to know what ram to give me. I did not have my laptop with me at the time, but I did show him the specs of the ram that is already in my laptop. He then took the ram that he recommended from the shelf and I puchased it. It did not work on my laptop. I took it back as soon as I had the chance. The technician said that my laptop is too old for the ram that was sold to me and that the ram required was discontinued. When I asked for a refund it was denied. I now have store credit that I do not want or need because the sales person did not insist on testing the ram on my laptop before selling it to me. He did not give me any indication that it might not work on my laptop. He also did not tell me that he will need more than the specs of my current ram to determine what I have to purchase. He just took it from the shelf and sold it to me confidently. I lost money due to a mistake on the side of the sales person and I would really appreciate a refund. I sent a message on the website last week but have not received a reply. I am very dissatisfied with the service!