I don't think Coinbase considers themselves a broker.
This is directly from coinbase's terms of service :
"3.1. Transactions with Coinbase. When buying or selling bitcoin through the Exchange Service, you are buying from, or selling to, Coinbase directly. Coinbase does not act as an intermediary or marketplace between other buyers and sellers of bitcoin. The Exchange Service is subject to the Coinbase "Conversion Rate." "Conversion Rate" means the liquidity adjusted price of a given bitcoin amount in terms of local currency as quoted on the Coinbase Site."
This is directly from coinbase's terms of service :
"3.1. Transactions with Coinbase. When buying or selling bitcoin through the Exchange Service, you are buying from, or selling to, Coinbase directly. Coinbase does not act as an intermediary or marketplace between other buyers and sellers of bitcoin. The Exchange Service is subject to the Coinbase "Conversion Rate." "Conversion Rate" means the liquidity adjusted price of a given bitcoin amount in terms of local currency as quoted on the Coinbase Site."