In many games, users have found a way of economy to obtain profit from the game by selling their accounts, and Clash of Clans does not escape these practices. Find out a little more about this matter in this Mobailgamer post that we made just for you.
Although it is true that when selling or buying an account there is a benefit, it is also possible that everything goes very wrong, since this activity is not monitored by the developers (SUPERCELL) who have not created an interface that allows trading the accounts, but which is done at the player's own risk.
As for the places where you can put your accounts that you no longer use for sale, there is Facebook. groups about Clash of Clans of this social network are full of account sale notices, it only remains for you to trust the seller or buyer that you get.
There are also people who use the Mercado Libre portal and some forums about the game dedicated solely to account trading.
When selling one you must take your security measures so as not to be scammed.