No, sites that sell kamas and/or items are not legal.
By buying items over the internet, you risk being scammed into purchasing goods that will never be delivered. You also risk having your Ankama account stolen.
Plain and simple, you risk having all of your accounts definitively banned.
How do I purchase items or kamas legally?
Ankama offers a very advantageous way to buy and exchange with Ogrines. These offer players a lot of flexibility and allow you to buy subscriptions and also in-game objects, kamas, or other services.
To buy Ogrines, you just need to go to the official DOFUS site.