Why do some members stiff me?

I`ve sent small things to members, paid the postage and still no thanks. I don`t know what people are thinking. On the flip side I`ve been the recipient of a small part from a member on this site and I offered to pay for the part and shipping and he wouldn`t hear of it. There`s both ends of the spectrum represented here.

True the same has happened to me. On the other hand, I sent a box full of water pump and crank pullies to a member, paid $9.87 in shipping and told him to send the ones back he didn't use. After months of excuses I gave up, and have never received my shipping money or the extra pullies back?