Well, costed in. And it is something that I'll always choose to pay for as buyer, because it does give me protection, versus F&F or a bank transfer with no comeback.
This is unlike postage, which is the seller's concern, not mine.
I'm absolutely fine with sellers doing the calculation of: money they want in their pocket + postage fees for their chosen method of sending + PayPal G&S fees.
Just do it up front and list that as your asking price. Don't get an offer then start considering postage costs, or sucking your teeth and saying "Ooh, you want to use PayPal, and you don't want to just give me the money without any chance of getting it back? Wow, such unusual, very unexpect, I'll need to work out the fees for that."
That's me, anyway. It may be the case that most buyers are happy paying F&F / bank transfer. I'd be fascinated to know, ideally from a prolific seller who doesn't have a clown in this fight.