There are times when you will get angry with a customer, it happens. Giving their anger back at them will only make the situation worse. Instead, try to find out the reason they’re angry (if it is not apparent) by re-stating their complaint this way, “I understand you are angry, just so that I understand why I’d like to re-state what you just told me, it that okay?” and then do it. This does two things, it slows down the anger process and it helps make clear that you understand their anger. Now ask them how they would like you to solve the problem. If you can solve the problem, do so, if you can’t, refund their money with a smile and invite them back next time they’re at the market.