Builders and Rolux Passing the buck

Purchased a Rolux Petrol Chain saw for minor maintenance on garden on the 26/11/2015. Within 10 minutes of FIRST use machine stops working therefore called in a garden service to finish off the job required to be done before a fence went up. Took the machine back to builders and explained product is faulty and no longer require it as CPA states a consumer can do so for defective products within 6 months( this was few days after purchase). Builders supervisor explained they need to sent machine for assessment to clarify if negligence or defective product was the cause before refunding me. On Assessment the machine was found to have a burned/damaged spark plug which is clearly a defect on suppliers side as machine was used for 10 minutes. Rolux wants a fee of R175 for a defective part used by them. Builders manager said they would replace the part as i refused but my point is that they promised me a refund or credit note if the fault was not due to negiligence. Builders is only a middle man that buys from suppliers and sell to consumers therefore they should do more to protect the consumer especially when the product is defective.