Buying a second hand mobile phone is not a bad idea at all. But you have to be careful with a few things as there is a high chance that you might be cheated by giving a improperly functioning phone or the inside parts may be changed and duplicate parts might have been put.
Certain thing that you need to keep in mind before purchasing a second hand mobile phone:
1) the seller-the most important of all of the seller should be reputed and trusted.
2) Always buy in a store, there are several shops these days in malls and outside and these sellers even provide warranty for a year or two.
3) Check the outer physical appearance for any damage.
4) Battery and battery life should be checked as the battery might be old and you might not be getting enough battery back up even for half a day.
5) Take a proper bill from the seller.
6) Make sure the accessories are in a good condition and in a working condition.
Well, be careful, check the phone properly and adjusted buy from a trusted seller.