I don't understand the outrage. You said it was $15, he said he'd pay $10. What's the problem? You have three options 1) accept the $10 2) counter (e.g., split the difference) or 3) politely say, no, I need $15 or it's not worth my trouble. Simple enough. If people making a counteroffer is that offensive to you, I'd suggest opening up the conversation with "I actually have the part, it's a pain for me to get to the post office, so I need at least $15 to make it worth my while" Then you've established the fact that it is a 'take it or leave it' situation.

(I admit it, guess I'm old....I just don't understand the moral outrage people feel over things these days, we're not talking about people's lives here)