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

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. 

No.

Seller seems like a jerk if it's as you say. You honored your end, seller needs to do likewise. Report extortion to A'gon