I come across sellers all the time who add a percentage to the sale if the buyer wishes to pay by Paypal.
It's against ebay rules because the seller is offering it as a form of payment. It is the most efficient and secure form of payment due to the insurance that you get with it and the auditable trail you get.
When you offer a service of payment to someone you cannot then ask them to cover the expense of it, I offer payment by cheque but I'm not gonna add my petrol money on or bus fare to the nearest bank so I can pay it in.
You may find your buyer reporting you and having your item taken off or have your account suspended.