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

I also agree that it was the seller's responsibility to disclose his terms BEFORE the deal. Whacking someone with additional hidden charges after the fact is not an honorable negotiating tactic and holding the item hostage smacks of scumbaggery. You would be within your rights to demand he ship the item as defined in the ad, and if he doesn't like it, then it'll be a valuable lesson learned on his part to be more diligent when posting an ad.

At the very least leave neutral feedback for the seller alerting future buyers of this practice. If the seller did NOT mention Paypal fees in the ad, I would never pay them. 

@parabolic Paypal would reimburse you the fees if you challenged and documented the seller's late request. 

I have both sold and bought on Ebay for years. It is the seller's responsibility to have all expectations of the buyer's payment included their ad.

The fees are actually the responsibility of the seller, and the seller should include anything they expect to recoup in the price and the shipping fees -- including any tax. If it is a foreign seller, they the buyer has a responsibility to pay any fees necessary for satisfying customs, etc., but I think this also needs to be explicitly explained and plainly documented within the ad. In my ads, I plainly have all that I expect with regard to the buyer's responsibility right in the ad itself. This will eliminate misunderstandings and hard feelings.

This also includes any fees that are charged to PayPal if that is being used. If Ebay or PayPal held you responsible for those fees . . . you would be charged directly by them. If you have already paid half . . . you paid too much because they are not your responsibility.

If this cannot be settled -- contact Ebay customer service and allow them to arbitrate.  Hope this helps.
http://ocsnext.ebay.com/ocs/cusr?query=597

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.