@muradi Bhai always prefer a new set over used one.Buying used mobo from market is a gamble,if any of your friends is willing to sell s4 or any set in your range that good otherwise its my sincere advice buy Qmobile noir i12.For 20k you are getting a brand new set with 720p ips hd screen,16GB rom memory extendable to 64GB,2GB ram.13MP rear/5MP frontcam 4.4 kitkat..Thats the best you can buy in 20k range..
for 20k or below range always prefer a Q as is offers more punch as compared to other premium brands..For 30k and above follow premiums.
For used sets LG G2,S4 Black,Xperia Z..As you are prefering resale other members can guide you better cuz i am more of a buy and forget sorta person as i keep mobiles for a long tenure.