Selling tokens is not difficult at all. Difficulties start when it is concerning price.
Tokens must be held for a decent period of time just to approach to pre-sale or ICO price.
Most of them will never do it so it's up for you to decide.
Where to sell? Open CMC and see the exchanges tab.
For how to know which tokens are not on the exchange how? and how to sell the token how? honestly I have never sold my token once