if it is ok for you to buy used ones you can in to chirag ali lane near grammar school at abids. it is the best place to buy mobile phones. you will not only find used ones but also new ones at very good rates. its a grey market so be carefull.
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not that they will cheat you or anything but you may end up paying more. i will suggest have a look around the place and inquire the price in a couple of places and then take the plunge. also you may find some fancy phone at some irresistable price. eg i once bought a nokia 7710 for only 12k when the market price was more than 20k and it was hardly old