Is it legal?

Yes, as long as one does not misuse it for illegal purchases. There was a case when bitcoin, due to its semi-anonymity, was the main currency accepted on Silk Road, the Dark Web marketplace for drugs and other illicit goods and services that was busted by the FBI in 2013.

Ever since, the US legislative has turned askance towards the trend and takes it under more scrutiny, respectively to its accumulating growth. 

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