What should a person who has defrauded people in sales do, when he now lacks sufficient funds to return the rights to their owners, and how should he deal with a non-Muslim person whom he defrauded by selling counterfeit goods as original?

For those whose deception you were directly involved in, you must return their rights to them. If you do not know the exact amount you defrauded them of, then you must estimate it based on what is most likely and give it to them. As for those who are unknown, their money must be spent on the welfare of Muslims, such as mosques, bridges, and the like, or given as charity to a poor person. If you are unable to return the rights to their owners, it remains a debt upon you until you are able to do so. If you die in that state, having exerted your utmost effort to deliver the rights to their owners, then we hope that Allah will forgive you.