When does seller's responsibility end?

A few years ago I sold a Phono Cartridge here on Audiogon . The Buyer took 2 weeks after receiving it to mount and use the Cartridge . He reported that one of the mounting holes was damaged and filed a report with Audiogon . Audiogon took my side and thought the Buyer took to long to test it . I had paid with a Postal MO .

That said , Paypal is a different matter . They always side with the Buyer .

Good luck !!

I shipped a Ref 5 to a buyer here on Audiogon. A few months later as we were emailing about our respective systems, he mentioned for the first time that, when he unpacked the unit, he noticed that the perspex top plate had cracked around the screw that holds it in place and asked if I would consider replacing it --about $125 from ARC. I had a good feeling about the buyer based upon our discussions. Too late to make claim with fedex. I replaced it anyway. I believe that we should all operate with an assumption of trust about a buyer unless his rating says otherwise. I think the buyer should be made happy unless he is a jerk, a thief or his claim seems fishy based on the seller's state of knowledge. That being said, under the same circumstance, if the defect was obvious as this one was and the cost of the request was disproportionately high relative to the price at which the item was sold, then that might have caused me to suspect that the buyer broke it and I might have responded differently. In other words---fishy.

Never buy anything without a CC or some type of recource. If the seller won't budge, leave them stuck with the item. There will always be another you can buy.

I think Brownsfan's comments cover things pretty well.
I would add, we all bear some risk when trading (both buying and selling) used goods. That is one reason I state in my ads for more expensive items that I will not trade with those having multiple negative feedback, and request conversation from those with no feedback or with one negative feedback. I think a person's experience and history in buying and selling shine a light on how they may respond when things don't work out perfectly, such as with the USB port. Just because someone has cash doesn't mean I have to trade with them.
As the seller, I would look at two solutions;
1. The buyer pay to ship the unit to an authorized Naim repair facility who can then evaluate it and report back as to whether there is in fact a problem and what repair costs will be. If there is not a problem then you are done. If there is a problem, seller should pay for repairs and shipping so the buyer has a functioning unit.
2. Buyer returns unit for a full refund.
I agree it has taken too long but if you accept PayPal for payment then you are stuck with their rules. These are the risks of trading in used gear. Fortunately, most of those we trade with here are honest and rational about things.

BTW, I failed to mention that the buyer in this case had one negative feedback. I overlooked this when offer was accepted. This is a quote from feedback:

"bad buyer, Accepted the deal and did not pay as promised,after one week waiting, just left a message to notice her financial emergency, and caused me a big loss for the listed item. Do not make the deal with this person."

So I guess it is my fault selling to this person in the first place.
Will have to see how it plays out.
I will definitely state in my future ads the time limit for returns.
BTW, I am not a dealer, just an enthusiast.