Banks have been known to claim unauthorised transactions when the cardholder goes over their credit limit. And the cardholder is not told. Which makes sense as the cardholder is not the one who ends up paying.

So rather than the cardholder being held responsible for spending money they don't have, or the bank being responsible for allowing such a transaction to proceed, the seller gets shafted.

I believe there is a $15 fee for an unsuccessful defence of a chargeback. For the untracked item, I would bend over - you have no proof the item was sent or delivered to the buyer. The other one, you should win as you have Paypal seller protection.

I would also contact, at least, the untracked buyer and suggest they might like to repay you rather than be reported to ACORN for online fraud.