supp**** with parts of wrong specification

I bought spark plak at midas, of which i had a right specifications and I gave it to the sales person in the ARB Midas in Herculean in Pretoria. Then I was supp**** with the spark plak by the sales person. When I fitted them they work fine for a limited period of time. Then my vehicle started loosing power when driven. Then when I went to check what was the problem in the engine especially with the plus I found out that they were damaged. Then I returned to the shop with plugs, I showed them to the sales person who was helping me. Then he checked on his computer and the manual, only to realise that the information feed on the computer was wrong the plugs i should have had was suppose to be short range while he gave me long range and the damage was already caused by the plugs he gave in the engine. But the sales person person said he needs to consult with his manager, when the manager came he asked me who fitted the plugs and he said if it was not a qualified electrician he won't be talking to me. The tone of the manager was full of bad attitude and ******. Please help