This is a very unfortunate situation. However, luckily, we aren’t talking thousands of dollars.
There’s a couple things that bother me. One is, the assumption that the guy you talked to is lying, a thief- because he’s nervous and Chinese. It’s reasonable he’d be a suspect because you sent it to his address, but, you cannot prove that he received it because he didn’t sign for it. You didn’t send it signature required. From what you said in your post , you have no proof if he actually received the package. An accusation , assumption, is not proof.
Secondly, it may not be your fault, but, it is definitely not the second buyers fault. The right thing to do is to immediately (if you are convinced that the buyer never received the item) refund his money. You have his money and he didn’t get what you sold him.
You’re the one who has to fight with Audiogon, PayPal, the shipper, the person you assume might have the item, etc.
Best thing to do is to refund the money to the guy who sent you his money. He never received the item you sold to him.
If you were the buyer in this situation, my guess is you would agree.