There have been some cases where people were charged with money laundering or running an unlicensed money transmitting business, and those cases involved selling BTC, but it wasn't alleged that selling BTC is illegal by itself.
FWIW the guidance has, since last March - May, suggested that if you are selling BTC for USD as a business, you must register as an MSB with FinCen. Additionally, you might need the appropriate state licenses depending on the details.
If you're just doing small trades as a trader for yourself, and not as a business, you might not have to, but I would certainly talk to an attorney!