IMO, if the seller as you said did not list in the AD that paypal fees must be paid by the buyer then you should not have to pay. It's really the seller's fault for not listing this fact and if the buyer presents this fact to the seller, the seller should be professional enough to accept his or hers mistake for not putting this note in the AD and not fight over it.
Seller asked for PayPal fees after the sale. Should I pay them or not?
I recently purchased an item and after accepting my offer the seller asked for PayPal fees (3%). I checked the listing and nothing was listed in the ad about my paying the fees. I have already payed half the fee but I'm not sure if I should have paid the full amount. Changing the deal after accepting the offer feels wrong to me. Looking for guidance.