It all depends on the tokens you want to sell. Go to the coinmarketcap - there will be all the information
But not all coins are listed in coinmarketcap because cryptocurrencies that doesn't meet the requirements set by coinmarketcap will not be listed in there. So if you are partcipating or investing in an ICO, the best thing to do is to ask the team in their social channel or telegram regarding where to sell their tokens.