Eldragon: I disagree with your statement "What you are paying for is supreme convenience." (in reference the seller).
As Bowow pointed out, it is actually "more" of a convenience for the buyer than for the seller. If I am selling something, whether you pay through PayPal or with a MoneyOrder is no big difference to me. As a matter of fact, I would PREFER the money order because:
A) it's money in hand whereas with Paypal I have to specifically request a transfer to bank
B) if you contest the paypal charge later, they dock me for it
C) while waiting for the money order to arrive, I could continue to enjoy the item (though I always pack it up right away)
I also don't buy the RetailStore-CreditCard argument. Retail stores are a business. They sell a LOT of things. The fees they "eat" from credit cards are built into their prices accross the board. ie. the NON-credit card users are subsidising the store for the credit card users. Given enough customers, it all evens out. HOWEVER, when it comes to a private party selling their $5000 baby, they can't be expected to "eat" $150, when the benefits to the seller are minimal compared to the benefits of the buyer.
Lastly, in an auction, the seller CAN'T "build" it into the price. The bid is the bid, and based upon the buyer's CHOICE of payment method, it should not effect the amount collected by the seller.
Now, if a seller lists paypal as the ONLY method accepted, AND adds the fee, THEN it would be a little obnoxious in my opion. In that case, find another seller - simple.