Crypto Trading 2017, page-23

Can someone please explain to me why there is a large price discrepancy between platforms.

For example, on Coinbase, Ethereum is trading at $335 and on BTCmarkets it is trading at $360.

Now i understand that Coinbase has a 4% trading fee, whereas BTCmarkets only has a 0.85% trading fee, but that still doesn't add up.

Does each platform have its own price? And what is stopping someone from buying on one site and selling on another?

Thank you,
L