I'm not entirely sure what the law is but would it really make a difference what tax you pay depending on how you sell it?
If you want to keep it legal then selling it for cash one way or another would mean you'd need to report your earnings and pay any consequent tax on that.
If you want to avoid unnecessary fees though you can post on localbitcoins and sell BTC in person for cash.
You remember bitinstant?
They tried to do this on a large scale and got caught in laws of us.
Nobody can do this because of taxation and money laundering laws.
It is not just not possible.