for smartphones, you want to go to the phone market in DaShaTou - it's googleable.
It's good for "perfect seconds". if you make sure you know how to check out the model you are buying, you can get the best deals on new, quality models from there.
Also, it's a well-established market place - the market rate there is stable enough that you won't have to worry too much about bartering/getting ripped off (again, provided that you know what you are buying).
I bought a phone at a popular phone market down near by the main river. The phone stopped working after a couple of days.
I would just buy one from a REAL outlet. don't risk it!
Best advice, just go to HK. With the exchange rate it'll come out much cheaper than the mainland and you won't get spyware in the OS
Thank you :)