Is it alright to buy a deck of tarot cards for someone else? I do not believe in them, so is it okay if I bought it for a friend that would like them for their birthday? Or am I encouraging something haram, even if they don't share my beliefs?
By the way, I am assuming your friend is a non-muslim and a male.
Why not give them something Halal for their birthday? For example, let's say you go to get those cards, and you went to a sketchy shop. On the way there you spot a drink that says "coke", you grab the cards and get the drink. On your way back, you drink the "coke" and it tastes good. And you give those cards to the person.
Now the other day you want another of those good "coke" and go back. This time you spot something haram and shaytan takes over your thoughts. As you are getting the drink and all, you seem to also lost your mind. Then they tell you at 2 PM Friday, they have "parties".
So on the coming Friday, you miss your Jummah prayer and go to the "party".
Now my point was not to guide anyone the wrong path, but I was showing that those cards you may want to get, may cause you to walk off the wrong path. Either you get into those cards or with the person you are giving them to or where you got them from.
All I did is portray a scenario of you getting the cards, and falling off Islam right path.
Now let's change the scenario a bit.
This time you decide that you want to give your friend a dinner. You take him out to a Halal restaurant to introduce to him Halal food. Then few days later he tells you he wants to go back to the restarunt, but this time you take your family. He notices all the other Muslim families there, and the respect and all going on, and starts learning the Islam religion.
Few months later he converts to Islam, and on his next birthday you give him a Quran.
So basically in conclusion, I just portray to you the Islamic path but visually. When you go get haram, you end up addicting to the haram.