It is legal for any person who sell his/her bitcoins, they have full right and full control on their bitcoins and they can do what they want with their bitcoins, by the way the better choice will be that do not sell bitcoins and use your money in the shape of bitcoin, so that we make it a successful currency.
yes there is no doubt about this that a person has full control on his bitcoin either he want to sell it or buy it or doing any thing else with it, but it is also a fact there you have to obey rules and regulation of a country where you are living, so if a country has banned bitcoin it is not legal to buy or sell bitcoin in that stat and still you are selling or buying bitcoin there then it is really illegal and you are are doing an illegal activity according to the rules of the state.