I don't really get it. Often some people here on DP as well as from other places tend to ask PayPal fees beside their actual price for any service/product. The thing is that they already get the business from someone then why would you ask for additional fees? Aren't those fees responsibility of buyer since PayPal is charging YOU to use their system. Seller may already have to pay fees for currency plus credit card. This is insane.
It's a really bad practice. The sellers want the fee, because they have to pay a disagio (a fee) off every transfer they receive. But clearly they should not ask for compensation of that from their buyers. The system still works, because the buyers are paying the additional fees. I am not sure whether the PayPal terms forbid additional fees. Most credit card processors do, yet many companies charge extra for using a card as a payment method - and get away with it. The payment processors probably don't want to lose business and silently accept the practice.