The way I look at it, whatever parts starts "acting a fool" if both people are trying to be fair and not scam one another, is instantly in the wrong. A little bit of friendliness and basic communication skills can resolve 99% of all problems when both the seller and the buyer want to make things cool
Case in point...sold two PS1 games, and apparently one game had a few more scuffs than I remembered / saw when I shipped. Buyer was kinda bummed, and within twenty minutes of PM's, everything was resolved, he was happy, knows I treat people fairly, and we all go on about our day no problem
If you want someone to help you, don't bite their damn hand when they try