With the flash ETH crash and Coinbase / Gdax often going offline I don't know why anyone still trades there. There are a ton of sites that have high volume. Great charts, And many ways to convert USD to Bitcoin or other currencies to trade.
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I like Bittrex very much.
I use that as well and a few others.
Best of luck. I'm using Poloniex, but I'm new to trading.
Poloniex is very slow at verifying your tier stages and has really bad support. But it is really nice for trading.
If you want to buy currency with fiat (€,$,etc) try kraken.com.
Poloniex isn't allowing withdrawals for Steem dollars.