When the US collectors trade, we just assume shipping to each other is equal, so we don't bother charging it to each other.
As far as my sales go, someone asked me how he knows I am really helping others with the proceeds. Well, my positive feedback shows that I am truthful, as the buyer could always be asked where he sent money. Otherwise, I am fine with a buyer writing a money order to the Salvation Army, an excellent charity. But if I trade, then certainly I benefit. Chalk if the first billiard collectible antique item I ever found....although I didn't consider myself a chalk collector, my collection grew so much that I have to consider myself that. So I have been spending more on the super-rare stuff lately, as it can only enhance my collection. I shouldn't sell the rare stuff, or I won't have anything to trade. So I keep one extra of many chalks.