If it were we wouldn't have market makers. There is an element of an auction...but its order driven ultimately:

The biggest benefit of an order driven market in liquid markets, is the large number of traders willing to buy and sell the security. Generally, the larger the number of traders in a market, the more competitive the prices become; this theoretically translates into better prices for traders. The downside to this type of market is that in securities with few traders, the liquidity can be poor. An example of an order-driven market is the Toronto Stock Exchange (TSX), in Canada.

Read more: An Introduction To Securities Markets http://www.investopedia.com/articles/stocks/11/securities-market-introduction.asp#ixzz3sAKc5Gur
Follow us: Investopedia on Facebook