I managed to solve the problem, so this is basically just sharing some experience.

I have never played WoW, when it was released on 2004 I have far less free time. Everytime there's a discussion about an mmorpg, WoW always get mentioned, so for curiousity sake I downloaded the client and created a character.

On the next day, i check this forum and I cant write any post. My avatar changed to the WoW character, despite that i have Diablo 3 standard edition, It said that people with starter edition cant write on forum.

When I login to Diablo 3, I couldn't access RMAH. It said "No Payment Method", I decided to delete my character in WoW. In 6 hours, I can write again in forum, My avatar back to the generic Diablo 3 avatar. However, I still couldn't access RMAH, so I top up $2 to my battle net account, it take roughly 36 hours before it's cleared, but now I can access RMAH normally again.

That's it. Thanks for reading.