Unhappy buyer .What to do ???

I would first question the one that told him that. was it a professional shop? Did they run a pattern and check backlash and rotating torque? "...been doing it for years" and "feels" are all indicators to me that the guy might not be a professional in which case it's a simple your opinion vs his. You didn't charge him a lot. I'd ask for the empirical proof. Having been a professional I wouldn't say "it's not visual but they're only good for a spare". I'd present him with measurements and a pattern and say the only way you can tell if there's noise is run them.