I agree with others that the refusal to repair or exchange a cable purchased used from another person is standard for most companies. Warranties are built around the notion that someone builds it properly and is willing to stand up for that. An intermediary could have damaged the cable and unscrupulously sold it to you knowing it was damaged. Or, they may have not liked it (and not known why) and not realized it was defective. In any event, I would not expect that asking a manufacturer to repair it or exchange it would work. I do think that repeatedly asking would eventually lead to a somewhat terse response closing down communication (since you are asking a lot from him but have never given him money for the product in question).
So the title "Buyer Beware" should probably apply to those of us who decide to buy used equipment here, and not to those who are seeking to purchase equipment directly from Mad Scientist Audio. I do not think that this company has acted unethically and am a bit surprised that an experienced audio buyer would think that this company owed him anything in this situation.