I travel to Hong Kong a few times a year but I am usually in such a rush I never as much free time as I would like.
The locals will all tell you to stay away from the Nathan Road shops as they are ripoff artists. I don't see much used anyway.
The one shop which I have visited with fixed prices on new items (very competitive to B&H of New York) is Tin Cheung Camera which is on Kowloon just off of Nathan Road. It is not a ripoff tourist scam place. They have an upstairs with used equipment. I never asked about their used equipment prices so I don't know how competitive they are. However on new items, Canon E0S lenses, point n-shoots the prices were very good and NO tax. International warranty. Here is the link to their site and you can click on the map to get better directions (the dotted line shows their old location, they just moved, the current location is the one on Carnarvon Rd). They sell new Bessa, Leica etc. Upstairs I saw a bit of everything used.
http://www.tincheungcamera.com.hk/
I hope others can chime in cause for sure I will be back in Hong Kong next year. The other shops recommended to me are all on Stanley Road on Hong Kong Island. My free day is always Sunday and the shops are closed! Tin Cheung is open on Sundays.
Another recommendation I got was:
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Photo Scientific
6 Stanley St, Central District
Tel: 2525 0550
Website:
http://www.yp.com.hk/photoscientific
The place to go for film stuff, excellent range of films, photographic
papers, darkroom gear etc. The other shops on Stanley Street are
mostly quite good too, but sometimes a bit more expensive than the
Kowloon-side shops. Tin Cheung Camera Company
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Closed sundays so I never visited. I was going to start at Photo Scientific and then ask/look around for other shops nearby.