Is the wholesale merchant sinful if he knows that some of his customers are defrauding his goods by selling them with higher specifications or at more than their real weight?

If you know that the buyer will cheat in selling the commodity, then it is not permissible for you to sell it to him, because that would be assisting him in the forbidden cheating, as Allah Almighty says: "And do not cooperate in sin and aggression." The condition for the permissibility of selling a deceptive item is that it be sold to someone who will break it or not use it to cheat. It is disliked to sell it to someone who is not trustworthy not to cheat with it, and the sale is annulled with someone who is known to cheat with it.