I like to swap for something I can use, but will help out a POOR kid with a freebie, if they're polite. (politeness is more important than wealth to me)
If I can't swap, I tend to sell with very "affordable" prices. I recently had 4 generic (cheap) MB knobby tires on CL for $2 ea. A guy showed up, driving a rather old looking car and wanted all 4. We talked a few minutes, and you could tell that he was proud his kids would rather ride their bikes than play video games! Even though I could use the extra $3, he got them for $5. He got a super deal and I got rid of them. Win-win!
I've benefited from similar situations, where someone else was willing to give a fellow "cyclist" a break.