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.
may be some countries have imposed such kind of law in their country that if one want to trade in bitcoin or do transaction or using bitcoin for any other reason he has to make a license for it as we normally do so for other things, but may be such kind of law is applicable in some countries and not every where because we have no such kind of restriction and we are using bitcoin with out any licence or any thing else.
It is true that we are the owners of our bitcoin and we have full right that we use our bitcoin, sell our bitcoins or hold our bitcoin. and the case that in some countries there is a ban on bitcoin and its usage, the people still can use bitcoin with freedom as bitcoin is an anonymous currency and no one know that who is holding bitcoin and who is using bitcoin.