What happens if a trader disagrees that an item is faulty?

I bought a pair of headphones from an amazon trader about a month ago that have now developed a fault - the trader has said I should return the item to them for testing - if they agree they are faulty they will replace them but if they don't they will charge me a restocking fee. The item is definitely faulty but what is to stop the trader simply denying there is a fault and charging me to replace them?