I sold a CAL Aria Tube CD player and buyer claims a burning smell

Only one question to answer. Did the buyer, receive what they paid for?
Was it received in the same condition that it was sent?
In other words, was it packaged properly by you?
Not did, "the shipper smash the box"?, BUT it was intact, not tossed around because of poor packaging. THAT COUNTS..

If you took care to make sure the buyer got what they paid for. You filled your obligation.
Stand your ground... "There is no cryin’ in Baseball!!!", a deal, is a deal.

Besides, it's close to time, to feed the chickens.

Regards

My story, saw a pair of Museatex Meitner monoblocks which come in beautiful rosewood cases on ebay. Answered the ad and seller texted me his phone number from his workplace, such email address having the name of his company. We talked and he said they were in perfect condition, having never been repaired. Received the amps and it was obvious that one of the amps had got so hot the lacquer had melted and then hardened at the bottom, hanging like small icicles. I opened the unit and totally obvious, repairs using different color wire, very crudely done. This amp ran twice as hot as the other.  Called the seller and he literally said "tuff luck, you bought them."

As background, in the early days of online emailing you were basically forbidden to use the work email for personal emails (probably same now). I then emailed him using his workplace email address and said that because he had used his work email I assumed his company was guaranteeing the product and then said I was about to call the president of the company whose name I mentioned. Within literally 20 seconds the funds were sent to my paypal account. I was stuck for the shipping both ways. Then found a beautiful pristine pair of the amps.

If the drawer isn't opening/moving, it could be because of a faulty (cooked) motor driver IC (or a component in the PS). Maybe you didn't detect the smell at the time, being in a well ventilated space. Now the unit's packed in a box for several days during shipment, and the smell might be detectable once opened.

I don't see how it matters, though. If I purchase a non-functioning item, it is guaranteed to have some failed components, mechanical, electronic, or electrical. What is unclear about that?

That's his wife's hair burning because he spent more on audio gear than he spent on her. I had one of those once.