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
Commit a felony in tax evasion and then go on to likely get scammed in the process. Going p2p with any significant amount of funds is going to leave you broke or dead. Go to an exchange and pay your dues.