You have to be careful about what exchanges you trust - there are many out there that are just dodgy, some are just outright scams. There are many examples of exchnages taking people's money and running. There are many reputable exahcnges though but remember even size of exchange doesn't nesccesarily guarantee safety.
now there are many new exchanges and some of them are scams, this is scary. try to find the best information in the forum or friend references about credible and good exchange. for trading now I believe in binance with a large volume and easy to operate, while to sell tokens from ICO results I often sell them on forkdelta, IDEX and mercatox