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.
When it comes to trading altcoins it is already old school on making transactions with other users here on this forum. Since exchanges are there we would normally choose up to sell or trade our coins without any worries of getting scammed just be sure to stick into reputable ones than on shady new exchange sites. Best platform would really depend on what exchange would suit your need there are lots to choose from but i would prefer bittrex and poloniex which knows one of the famous and reputable exchange as of now.
I agree with you, Poloniex and Bittrex are reliable exchange but have a fairly large fee.
do not forget Cryptopia, Binance, HitBTC that the exhange is a trusted exchange after poloniex and bittrex.
Plz does Cryptopia, Binance, Hit have lesser fees? Which other exchange has lesser fees?