For your first sentence, the correct way to say it is: "I sold a pair of pants that were the wrong size to my customer. She just left the store." If someone tells you that you are so hard to understand in English, you could say to them: "I am sorry, please be patient with me. I am just learning English. It is not my first language."

I sold a pair of pants to a customer in a wrong size. She left the store. If someone tells you that you are so hard to understand in English, what would you answer?