Coinbase has a trading platform, gdax.com. If you wanna just trade BTC, ETH and LTE, then coinbase will let you transfer money or coin onto the gdax platform for free. Just go to gdax.com and sign in with your coinbase credentials. Then click the deposit button, select coinbase wallet and amount. DONE. Now you can buy and sell without being charged the coinbase commissions and spot price premium.

FYI-
If you deposit money in USD into coinbase, there is no fee. Then transfer onto gdax, there is no fee. Then buy crypo, SUPER SMALL FEE. Then transfer new coins back to coinbase wallet, there is no fee. Now you can basically buy and hold cryto on coinbase for FREE! Coinbase exchange is good. I use Coinigy as my interface though because I don't like how gdax looks. Also because QuickfingersLuc uses it...