I think when a buyer requests a partial refund for goods purchased on grounds that are not based on what a sellers knows to be true then negotiating out of the situation only has limited value, unless you are prepared to give the partial refund.

I just think there is a time in transactions such as these that a seller needs to move from "make the buyer happy, even if I am out of pocket" into "protect my trading reputation and minimise the fallout" mode.