Why would you sell that coin for Ethereum if you will just sell it again to acquire some Bitcoin? exchanges will just charge you more fees if you do that. What exchange do you use for trading? if you are using an exchange that have a trading pair for Ncash to Bitcoin directly then you can choose that, as a trader you should know that every trade you make in cryptocurrency exchanges there is a fee or maker taker fee. So as much as possible avoid selling your altcoin for another coin just to buy another coin, unless there is no option that you can buy/trade a specific coin using Bitcoin or any cryptocurrency you own.