I'm going to sell overpriced Magic: The Gathering decks to a classmate so that he won't spend his money on drugs
Why would you rip a friend off like that?
I'm just giving him a better way to live in distorted realities
Even if you think it's keeping him from drugs, ripping your friend off like that's not cool. There's better ways to keep him from drugs.
His life, his choice. He has to learn from a messed up mistake.
I agree that your friend is making a voluntary choice to buy from you, even at inflated prices, but it is still wrong for you to take advantage of him financially.  Maybe try to offer him more decks at a fairer price, and even play the game with him to keep him even further away from drugs.