As others have stated, it's not just how one asks. It's how one replies too.

That ninja asked, "Hey, can you pass on all healer gear?", which is perfectly fine and not rude at all.

However, you replied, "No, I'm not going to pass on anything." That reply makes you seem like a jerk. Are you refusing because you just don't want to accommodate his request and being a jerk or did you truly need the healer gear too?

You could have replied much more politely like, "Sorry, I need healer gear too". If he was rude after that, then it was his fault.

Sounds to me like in this situation, you were the first one to show hostility and hence you should take part of the blame too.