Why do you even have to use alt coins. You will find many traders here who would like to trade in btc but also ALTs if they need it. Best way is to just convert your ALTs into Bitcoin with the help of bittrex for easy trading. Also you will find trusted member here to trade with rather than just trading with someone you don't know anything about.
Trading altcoins does not necessarily mean doing a direct P2P trading. If the OP is looking for a platform he can do pro trading, either day, short or long, then he cannot be doing that here on the forum and he will definitely need a good exchange to achieve his aim.
In that case, bittrex should be his best bet for a start but he should be ready to get his account verified with proper documents or else, he will get locked out.