I'm going to deny my cousin the right to sell his games for weed
What about the money your cousin makes that you don't like?
I think he is distorting the purpose of money to buy drugs.
While you may not agree with your cousin using his sale proceeds to get weed, it's not your right to deny his right to use his property as he sees fit. Your cousin can make his own decisions.
I'm going to teach him a lesson, I'm going to tell my uncle that he wants weed.
It is wrong to deny people doing what they want. If you take away their things they might resort to stealing to fund their habit.