|Focus on the problem, not the person.
Start by assuming your trading partner is doing their best. Problems can often be the result of simple miscommunication. Anyone can have an emergency, or a payment problem that's not their fault.
Make it clear to your trading partner that you see the situation as solvable, and that you don't assume they've acted with bad intentions. They'll usually respond in kind and you can resolve the problem quickly.
Talk about the situation you want to resolve and potential solutions, not what you think of your trading partner. Insulting your trading partner almost always makes the situation harder to resolve.