It is upto which country you are staying, because each and every country have their own law for bitcoins, in my country still bitcoin is not legal or illegal, so we dont give any information. or we say it as short term profit.
yes it is totally depending on the policy of the state, if a state has consider bitcoin as legal currency then you can buy or sell your bitcoin but your state has consider bitcoin as illegal currency and they are not allowing people using bitcoin then it is illegal for you to buy or sell your bitcoin, i think still most of the countries have not yet consider bitcoin as legal currency but still people there using bitcoin illegally.