For discounted designer stuff (not second hand though, just marked down brand new items), check out also ISA Boutiques in HK. I remember there's one along Haiphong Road corner Canton Road in Tsim Sha Tsui in Kowloon. It's located in front of Silvercord Center. Look up for the sign, you have to go up a flight of stairs.
Oh yes, yes, I forgot about this one. They carry brands such as Gucci, Burberry, Fendi, etc. There's also one in the China - Hong Kong Terminal in Tsim Sha Tsui Mall's Basement.