|Focus on the problem, not the person.
Start by assuming your trading partner is doing their best. Usually problems are a result of simple miscommunication. And 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 resolvable, and that you don't assume they've acted with bad intentions. They'll usually respond in turn, and you can resolve your problem quickly.
Talk about the situation you want to resolve and potential solutions, not what you think of your trading partner's character. Insulting your trading partner almost always makes the situation harder to resolve.