When you place a market order through a broker you are charged fees for initiating a transaction. What most people don’t know is that by placing a limit order you can bypass those fees.
All you need is an account through coinbase (see How to Buy Ether (ETH) for details) in order to place orders through GDAX.
Log in to GDAX and switch to the “Limit” tab in the left hand column. Enter an amount a bit lower than the current going rate for a coin if you’re buying or a higher amount if you’re selling. Make sure to select the appropriate “buy” or “sell” tab. If you want to add a safety net for yourself you can open the “Advanced” section and change the time in force policy to Good Til Time or Fill Immediately options. Those options will result in the order cancelling itself if it isn’t filled in an amount of time you are comfortable with. Otherwise the order could sit unfilled indefinitely which would leave you committed to a purchase or sale until somebody agrees to the price you set.
After clicking the button to place your order it will appear in the order book to the right. The order book displays the active limit orders. When somebody places a limit order that matches your price the transaction will complete. For example if you are willing to sell your ether at 289.10 per coin and a buyer places an order at or above that price you will be matched. It will automatically chose a transaction that is at your price (or better if possible).