Thanks for reaching out to us,
By "sell their in-game account", I presume you either mean physically selling their account, or gifting villages.
If you take a look at our rules, which can be found here:
https://www.tribalwars.net/en-dk/page/rules
§6 Account trading
It is not allowed to sell, buy or offer accounts or parts of accounts in exchange for Premium Points or any other outside benefit.
This rule, effectively prohibits the action of trying to sell your account.
As for gifting to the enemy, this is allowed and is an accepted way of playing the game.
I hope this helps, if it doesn't feel free to reach out to us in a support ticket, or feel free to ask further questions here.