The 1st thing Paypal asks is, "What was the seller's return policy?" When none is stated, none is assumed.
The 2nd question they ask is, "Has the item been returned?" If not, it's often difficult to prevail in a claim. However, if proof of return is evident, that in itself typically warrants seller return of Paypal funds.
Definitely sad to hear of this saga. Best wishes to OP in resolution.
A way to avoid the saga is to clarify terms and expectations prior to payment. For example, have the seller send a Paypal invoice for payment, stating that returns are allowed. If they refuse to do so it is a major red flag to a sensitive or astute buyer, and reason not to send payment, ending the deal before payment and shipping occur.
From a buyer perspective, if a seller refuses to agree on returns, deception or misrepresentation is possibly indicated.
The seller could clarify with additional ad wording "Sale is final. No returns accepted." Of course, such a seller statement is rarely included. It is a major red flag in itself, and enough to kill a deal.
Ironically, a prevalent legal concept is that used items are sold in sale terms of being as-is. No warranties, returns, or refunds are assumed, unless specifically stated, or strongly implied otherwise.
Let a buyer beware: here, there, and everywhere; past, present, and future. All of this is far beyond the world of Talkbass.