Most countries you have to pay tax off of your investments and if you use a centralized exchange that converts your crypto to fiat then you will have to report it.
To avoid this, just go straight to the buyers.
That's a terrible advice. Going directly to the buyers, like all the trustworthy parties involved... ahum
I also would not think of trading my coins from person to person, there is a lot of risk to that, I'm ok with the tax as long as my money is safe and I can withdraw it, even here in Bitcointalk there are scammers unless you do an escrow which is quite costly too.