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

I just can’t believe how some people would flagrantly lie and rig the system.

Believe it. There are lots of these people out there and they will do all kinds of things to cheat you out of money and/or goods. The advice above about getting your player back and issuing a refund is solid.

Try to learn as much as you can about who you sell to or buy from before you ship or pay, but even then there’s no guarantee that the transaction will go well. Most of the time you will deal with honest people, but there are many dishonest people on the internet.

Did it happen after he played it ?

Or were the tubes ready to go ? They can give off a burn smell then .

Take the unit back, you probably did not sell it for enough money that it makes a difference.  You can have the person send it to me for a look and repair as an option.  I can take a look and give you a repair estimate.  Or just take it back and list it for sale again.  It doesn't matter if he played it, it is not worth the money.  Have him pay for the return shipment and once you get it back without any other damage then refund the purchase price.

Just my $.02, but with the few unhappy buyers I've come across, I send a prepaid return tag and pay both sides of the shipping.  It's a small price to pay for the goodwill, and keeps your karma clean.  Make them happy and move on.

Though one can ascribe all sorts of chicanery to either party in a deal gone bad, it's been my experience that most buyers and sellers are pretty honest and straightforward. Disputes that I've been involved in have been more a matter of misunderstanding and lack of disclosure (my perception of "mint condition" may differ from yours) than outright dishonesty or fraud.
Your mileage may vary.  Best of luck sorting this out!