While I'd personally (in the extreme wide majority of cases) advise against day trading in general (case in point no. 1, no. 2 and no.3; see this post for one example on what to do instead), there's plenty of massive legit cryptocurrency exchanges (e.g. Bitstamp, Binance, Kraken etc.; while I haven't heard about major ongoing issues in any of these, obviously, do your own research, buyer beware and all that jazz) that you can buy / sell a variety of cryptocurrencies and be relatively safe that you'll be able to withdraw them without any advance fees.