BACS payments can be reversed weeks or months down the line if it turns out they were fraudulent.
A potential scam is they refund via BACS from someone else’s hijacked bank account. Once the person who owns the bank account realises they inform their bank and the bank reverses the fraudulent transactions. You lose the money and they’ve long since disappeared with your paypal payment.
I’d request they refund via paypal and can’t think of any reasonable reason why they wouldn’t agree if it’s all above board.
Even if they seem like a trustworthy ebay seller there are plenty of examples of scammers hijacking other sellers ebay accounts to run these type of scams so they appear more legit.