As a 14 year eBay seller, I've always added the fees into the cost of my items. But if you choose to pay with a money order, is it fair that you paid an extra dollar or so for your item from that built in fee coverage? PLUS the $1 or so you have to pay to buy the money order and stamp? It really comes down to personal opinion and you will never find that everyone agrees with only one or the other. There's arguments of pros and cons for each side, and as long as that exists, there will be no big final agreement.
Simple enough, if you don't like the terms, don't buy. If you buy and then decide you don't like the terms, that's YOUR fault, not the seller's. I can't tell you how many times I got screwed because a buyer wanted to try and change the terms after a sale. I have the item, you have the money, so it's your choice if you want the item enough to agree to my terms. Comes down to simple suppy and demand.
My two cents.