Coinbase acts as an exchange. ...
Lets say I want to buy bitcoins for 600/coin, so I post an "bid order", that tells people I'm willing to give them 600usd for a bitcoin. ...
Unless Coinbase has changed recently, that is incorrect. On Coinbase, you buy or sell directly from them, there are no bids or asks.
Coinbase gets their bitcoins from bitcoin merchants who want an instant cash-out to USD. In this way, it's an exchange where exchanging from USD to BTC is public and BTC to USD is reserved for bitcoin merchants only. Merchants may have a locked in BTC->USD exchange rate, which is why coinbase's btc selling price is higher than its buying price.