Mobile Phones: Most of the shops have decent products and good prices. You're not likely to get a deep discount if you're looking for Hong Kong a warranty. If you're willing to take a risk, ask for a phone with a shop warranty.
I've bought phones from Mongkok, Ap Liu Street, Central shops without much problem. At the end of the day, make sure there are several others buying from the same shop and you'll be ok... don't be the only idiot who has walked in.
Digital Cameras: Again, have bought from shops in Sheung Wan, Stanley Street and even Chungking mansions without a problem. If you know what you're looking for and know exactly how what a good price is then take the risk.