Do you have to disclose it? No. I would not even worry one minute about it. There are trademarks for everything and tons of overlap. You can't control what someone is going to do with their domain. They (like you did earlier) need to do their own due diligence. But, if you have some sort of pending legal action, you will need to disclose or risk having the sale negated (with possible penalties) from some sort of legal proceeding.