Seller asked for PayPal fees after the sale. Should I pay them or not?

If the seller didn’t ask for Paypal fees in the ad, then you’re under no obligation to pay them. If you paid half the fee...good for you, but you had no obligation to pay and the seller has no right to ask. That’s the simple answer!

Does Audiogon have a written policy about a seller adding PayPal costs when  the listing is silent about who covers PayPal fees?   

The deal is done. We call that balls up in my neck of the woods!

Next thing the loser will want is packing and fuel fees.

This happened to me on my first deal here on Audiogon. I was disappointed at the lack of integrity by the seller but since I had already made payment I paid his ransom payment. The seller's Audiogon account was later discontinued by Audiogon for some other nonintegrous behaviour with another member.
All fees should be disclosed upfront. If the seller didn't mention the fee in his ad it should not be your responsibility.