Happened to me in 2011 (13 years ago even before all the news about Chrome extensions being malicious etc). Sold my extension for USD10K. New owner modified it, inject tracking code & replace ads on websites with their own. I was very angry because it was unethical. I didn't know they could do that as they told me Google allowed it at that time. It got banned by Google a year later.
It was one of my best projects, but I don't feel comfortable talking about it anymore.