I don’t know too much about this. I only know that only if you purchase certain items from certain stores will you get a tax rebate. I am not sure if the stores at Guang Hua Market are part of the list. I know Sogo is one of the stores, though.
New York New York in Xinyi will give you the rebate if you are a tourist with your passport/visa - 2F near the lifts(elevators) in the middle of the back of the building. So when you have bought your size 5 electric pink boob-tube with diamante applique that spells “I’m a Bimbo” you can get your 5% back…
I heard someone say you can do it at CKS airport(???)