I have this in the top of my profile and I still can't get people to read it.
<TL;DR> - Do not use certified mail unless we have discussed it, the trade needs to be over 25 dollars US or 50 international, yes you can use PWE just don't cry about "but it was in X condition when I sent it" because that isn't my fault. If we are trading RL I will always ask for pictures, don't care if the card is a buck or 500.</TL;DR>
I've done so many trades I've only had 4 issues out of my over 600 trades, but I am always up front about expectations. Sometimes people say 'nope' and they are out, and that is great; because if we can't agree on the principle then I don't want the trade to move forward anyway. I feel as long as the expectation is set before accepting no one is ever upset. And times where my side has messed up, I make them whole, I don't want anyone having a crappy experience, and I've had the same done for me.