Is the buyer responsible to cover the cost of a return?

I’ve had a couple of instances with buyers and when I told them that if the unit returned to me was the exact item (serial numbers matched for example), then I would refund their money. None of them came back after that.How do you know if he received the speakers and he didn’t like the sound, he could be making up an excuse to get his money back especially when he claims only certain music has a problem. I would tell the buyer that if you get the speakers back and you test them out using the exact piece of music and there are no issues, then there would be no refund. If indeed there is a problem, you then have to determine if the problem is with the speaker or shipping caused this, not the buyer.This is exactly why my last 2 high budget sales, I listed them as local sales only so the buyer can make sure everything was as stated correctly in the ad, then a cash only transaction occurred. Both guys drove over 600 miles to pick them up (Hanns T-60 turntable and Usher speakers) and I had all the original boxes.  Wood crates won’t protect a piece of equipment if it’s dropped, something could become loose or damaged inside the unit

  low buzz does not + blown speaker. Probably never the case. It's much more likely he's hearing some sort of AC hum or noise from an upstream component, grounding issue. He should use a cheater plug on his component's and or run them temporarily off of an extension  cord connected to a different circuit- 99% the hum will go away and the speakers won't need to be shipped back. This one screams "user error"...

What shipping service what was used?
Fedex or UPS are the worst for any speaker shipment using original boxes or not.  If retail dealers use those companies they are shipped using pallets to avoid problems.