Is it legal to sell Bitcoin? (USA)

After looking around it appears there is no easy answer. I also wont be taking any responses as "legal advice".

I will break it down to a simple example.

Lets say A friend wants to pay me $500 for some bitcoin because they dont want to go through hassle of setting up an account on a exchange. Is that legal?

Please post links to sources because I dont think strictly opinion is helpful.