Thanks Drake and Madborius for your very prompt response and action.
To answer your question:
I have used the "report the post" function at the bottom of the sales thread on 22 July;
I have contacted 4 ASC staffs and received one reply directing me to the "report the post" function.
I am relieved that seller's AV status has been pulled, so no more ASC members should become victim like me until this matter is solved.
I would now resort to PMs to seek your kind help for my next action.