I have a simple question. I don't know If I'm allowed to refer a person who create graphics for another who wants to buy a graphic?
Thank you (:
You should ask permission or at least check if they are taking commissions out of courtesy, but it's fine.

As long as it's okay with the graphics creator and you're not shoving it down anyone's throats, it's probably fine. Referrals are okay, aggressive advertising and spamming are not.