If you are planning to buy a New Xiaomi handset then do check Mi Exchange. Xiaomi and Cashify have partnered to bring a new Exchange Program that lets user exchange their old working phone with a new phone from Xiaomi.
Now this exchange can be done at any Mi Home store, so far there are limited stores but Xiaomi is planning to open 100 Mi Homes in India in 2 years. There are two ways you can visit the Mi store with your old phone, they will give you the resale value and if that is okay you can use the money and pay balance if any for a new Mi Phone from the store.
The other option is to buy the handset from the Mi Store and get a pickup service of your old phone. But in this case you will need to pay the full amount of the Mi handset that you will buy. Then when the Cashify executive visits your place to collect your old mobile phone they will pay you cash as per the resale value.
The issue is the resale value that Cashify gives is not that good, you can get a much better deal if you use OLX or Quikr.