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

OP, you say you have already paid half the Ppal fee.
If so, and the original listing didn't mention anything regarding Ppal fees. Then, I think the seller should be gracious enough to accept what he has been paid-Even more gracious to refund you the fees you have paid.
I know sometimes I have made a mistake in a listing, but I always try to make thing right.
A Paypal fee is only 3%, so unless it is for a huge sum, then the amount should be something any reasonable person could afford to eat.

@gdnrbob "I know sometimes I have made a mistake in a listing, but I always try to make thing right" - that is what makes this site work, members treating each other with respect and always trying to do the right thing. I have dealt with many members here, good members, who also try and deal that way, many issues have arisen over the years and they always got resolved because in the end, everyone shared a common love for the hobby. Hope its stays that way, OP's seller not withstanding.

Mistakes do happen.  I listed 2 pairs of cables for what I thought was $100.00 each pair but my wording was poor and I realized I had listed the 2 pairs for the 100.00.  Buyer was smart enough to realize what I meant and I would have sold him both for the $100.00 as I didn't want any negative feedback on my first sale.  He paid for both sets and we were both happy.  Communication is key.

You are not obligated to.  The terms of the sale should be before the sale.

Personally, I think we should accept most buyers will be using PayPal and price our items with the understanding that we'll have to pay a fee. Guess what? Virtually every merchant you deal with who accepts credit cards pays a fee for that service. I never charge PayPay fees but do ask that people consider paying as a "friend" and I've done the same. No PayPal fee but less protection for the buyer. I have a 100% rating here and palm to keep it that way.