For a reliable store go to a "Fortress" or "Tai Lin" store, if you are in the mood for an adventure then get off the train in Mong Kok, step outside and smell the phones
This entire area is full of vendors.
A couple of tips: Check the box, check the IMEI stickers and check to be sure the battery is original. They won't just TRY to rip you off; it is a sport to them

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Large stores are all fixed price, smaller stores and vendors are very flexible.
Also; make sure to get 2 invoices