Given the fact that they took the time to write a formal complaint in clay (something that's expensive to do in the ancient world), it's indeed likely that his copper was shitty:
"He sold low grade copper disguised as premium grade. He is a man of low repute, a copper dealer with questionable ethics, who cheats people out of their money."
Or you know, just imagine that review on Amazon with 1/5 stars and way more expletives.
I always wondered what the circumstances were that the tablets remained where they were. Was the house abandoned after he died? Did subsequent people who lived there just keep the tablets around for some reason?
When you're a merchant so famous that people almost 4,000 years in the future and all over the world know about that one time you screwed over a lady so badly that she wrote you a strongly worded complaint.