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 the bitcoin you are holding is your property, but you are living in a state and you have to obey the rules and regulation of that that, and if you are not doing so its mean that you are busy in an illegal activities, and you will charge for that, therefore if a stage has consider bitcoin as illegal then all activities related to bitcoin will also be consider as illegal, therefore buying and selling of bitcoin is illegal in that state.