Unless it's changed Paypal does not charge the sender fees.
Perhaps they mean that they pay the fees, as the buyer, by calculating what the fees would be and sends the extra amount (the fee/s) on top of the price for the item.
Such as, when asked to send payment as a "gift", telling the seller that they would rather pay the fees instead of sending payment as a "gift". Also, some sellers request that the Paypal fees be paid.
Heck, one time I sent a payment, via Paypal, without including the Paypal fees (and seller had not requested that they be paid) and, next thing I know, the seller returned my money, & voided the deal, because I had not included the Paypal fees. Even refusing to do business, at that point, if I resent payment along with Paypal fees (since he was so angry that I had not done so in the whole beginning). If such a situation happened to someone else, I could see them just automatically paying the fees (without being asked) to other sellers.
I, myself, never request that the buyer pay Paypal fees and accept that it will come out, from my end (as a seller), since that is Paypal's terms.
Some sellers do request that the fees be paid and has been becoming such common place that many, in the reptile community, seem to have just accepted it.
As for the topic, of this thread, it does not seem to be a simple one.
There was the initial misrepresentation, which would have been enough to undo the transaction. However, the OP agreed to accepting supplies to fix the two damages mentioned by Sarah. This was prior to the OP receiving the item/s. Thus, the two disclosed damages were accepted.
Then, when the OP receives the item/s, there is, allegedly, more damage than what Sarah had disclosed (after sale and receiving payment). So, OP wishes to undo the transaction.
Bad thing is that there was a middle man. If the middle man caused the extra damage, then the middle man would be liable for those damages.
Yet, here we sit with very little proof, of much else or what actually happened, other than the initial misrepresentation and disclosure. Plus, no photographic proof, from the OP, of the extra damage/s.