that is total bullshit about "consumer affairs" mate and they never told you that.
if you are a registered business and you sell a product that is "not fit for purpose" you have to give him a refund.
it does not matter if it is self installed or not the same thing goes for many auto stores around australia.
that would be the same as bunnings refusing refunds because the majority of their products are aimed towards the diy market and yet they still have to give refunds.
the simple fact is you value the dollar more than the customer satisfaction, you should have given him a refund but according to him you made threats and if he was more clicked onto acl he would wipe the floor with you at a tribunal but one day you will find a customer that will know what to do and you will learn